TUNICATA. This group of marine animals was formerly regarded as constituting, along with the Polyzoa and the Brachiopoda, the invertebrate class Molluscoidea. It is now known to be a degenerate branch of the Chordata, and to be more nearly related to the Vertebrata than to any group of the Invertebrata. The Tunicata are found in all seas, from the littoral zone down to abyssal depths. They occur either fixed or free, solitary, aggregated or in colonies. The fixed forms are the “simple” and “compound” Ascidians. The colonies are produced by budding and the members are conveniently known as Ascidiozooids. Some Tunicata undergo alternation of generations, and most of them show a retrograde metamorphosis in their life-history.
History[1]
More than two thousand years ago Aristotle gave a short account of a simple Ascidian under the name of Tethyum. Schlosser and Ellis, in a paper on Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the compound Ascidians into notice; but it was not until the commencement of the 19th century, as a result of the careful anatomical investigations of G. Cuvier (1) upon the simple Ascidians and of J. C. Savigny (2) upon the compound, that the close relationship between these two groups of the Tunicata was conclusively demonstrated. Lamarck (3) in 1816 instituted the class Tunicata, which he placed between the Radiara and the Vermes in his system of classification. The Tunicata included at that time, besides the simple and the compound Ascidians, the pelagic forms Pyrosoma, which had been first made known by F. Péron in 1804, and Salpa, described by P. Forskål in 1775.
A. v. Chamisso, in 1819, made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described by J. J. S. Steenstrup in 1842 as “alternation of generations”; and a few years later Kuhl and Van Hasselt's investigations upon the same animal resulted in the discovery of the alternation in the directions in which the wave of contraction passes along the heart and in which the blood circulates through the body. It has since been found that this observation holds good for all groups of the Tunicata. In 1826 H. Milne-Edwards and Audouin made a series of observations on living compound Ascidians, and amongst other discoveries they found the free-swimming tailed larva, and traced its development into the young Ascidian.
In 1845 Carl Schmidt (6) first announced the presence in the test of some Ascidians of “tunicine,” a substance very similar to cellulose, and in the following year Löwig and A. v. Kölliker (7) confirmed the discovery and made some additional observations upon this substance and upon the structure of the test in general. T. H. Huxley (8), in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology and affinities of the pelagic Tunicates Pyrosoma, Salpa, Doliolum and Appendicularia. These important forms were also investigated about the same time by C. Gegenbaur, C. Vogt, H. Müller, A. Krohn and F. S. Leuckart. The most important epoch in the history of the Tunicata is the date of the publication of A. Kowalevsky's celebrated memoir upon the development of a simple Ascidian (9). The tailed larva had been previously investigated; but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not been understood. It was reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the vertebrate embryo. He showed that the relations between the nervous system, the notochord and the alimentary canal are the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration, and this led naturally to the view usually accepted at the present day, that the group is a degenerate side-branch from the lower end of the phylum Chordata, which includes the Tunicata (Urochorda), Balanoglossus, &c. (Hemichorda), Amphioxus (Cephalochorda) and the Vertebrata. Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by C. v. Kupffer (12), A. Giard (13 and 15), an others.
In 1872 H. Fol (14) added largely to the knowledge of the Appendiculariidae, and Giard (15) to that of the compound Ascidians. The most important additions which have been made to the latter since have been those described by Von Drasche (16) from the Adriatic and those discovered by the “Challenger” and other expeditions (17). The structure and the systematic arrangement of the simple Ascidians have been mainly discussed of recent years by J. Alder and A. Hancock (18), C. Heller (19), H. de Lacaze-Duthiers (20), M. Traustedt (21), L. Roule, R. Hartmeyer, C. P. Sluiter, W. Michaelsen and W. A. Herdman (17, 22). In 1874 Ussoff (23) investigated the minute structure of the nervous system and of the underlying gland (first discovered by Hancock), and showed that the duct communicates with the front of the bronchial sac or pharynx by an aperture in the dorsal (or “olfactory”) tubercle. In 1880 C. Julin (24) drew attention to the similarity in structure and relations between this gland and the hypophysis cerebri of the vertebrate brain, and insisted upon their homology. M. M. Metcalf has since added to our knowledge of these structures. The Thaliacea have of late years been the subject of several very important memoirs. The researches of F. Todaro, W. K. Brooks (25), W. Salensky (26), O. Seeliger, Korotneff and others have elucidated the embryology, the gemmation and the life-history of the Salpidae; and K. Grobben, Barrois (27), and more especially Uljanin (28), have elaborately worked out the structure and the details of the complicated life history of the Doliolidae. Finally, we owe to the successive memoirs of J. Hjort, O. Seeliger, W. E. Ritter, E. van Beneden, C. Julin, C. P. Sluiter, R. Hartmeyer and others the description of many new forms and much information as to the development and life-history of the group.
The new forms described from Puget Sound and Alaska have drawn renewed attention to the similarity of the fauna in that region of the North Pacific and the fauna of north-west Europe. There is probably a common circumpolar Tunicate fauna which sends extensions downwards in both Atlantic and Pacific. As the result of the careful quantitative work of the German Plankton expedition, A. Borgert thinks that the temperature of the water has more to do with both the horizontal and the vertical distribution of pelagic Tunicata in the sea than any other factor. It is probable that the occasional phenomenal swarms of Doliolum which have-been met with in summer in the North Atlantic are a result of the curious life-history which, in favourable circumstances, allows a small number of budding forms to produce from the numerous minute buds an enormous number of the next generation. The great increase in the number of species known from nearly all seas during the last twelve or fifteen years of the 19th century enables us now to form a truer estimate of the geographical distribution of the group than was possible when the “Challenger” collections were described, and shows that the Tunicata at least give no support to the “bi-polar theory” of the distribution of animals.
Anatomy
|
|
Fig. 1.-Ascidia mentula, from the right side. |
|
at, Atrial aperture; br, branchial aperture; t, test. |
As a type of the Tunicata, Ascidia mentula, one of the larger species of the simple Ascidians, may be taken. This species is External Characters. found in most of the European seas, in shallow water. It has an irregularly ovate form, of a dull grey colour, and is attached to some foreign object by one end (fig. 1). The opposite end of the body has a terminal opening surrounded by eight rounded lobes. This is the mouth or branchial aperture, and, it indicates the anterior end of the animal. About half-way back from the anterior end is the atrial or cloacal aperture, surrounded by six lobes and placed upon the dorsal edge. When the Ascidian is living and undisturbed, water is being constantly drawn in through the branchial aperture and passed out through the atrial. If coloured particles be placed in the water near the apertures, they are seen to be sucked into the body through the branchial aperture, and after a short time some of them are ejected with considerable force through the atrial aperture. The current of water passing, in is for respiratory purposes, and it also conveys food into the animal. The atrial current is mainly the water which has been used in respiration, but it also contains all excretions from the body, and at times the ova and spermatozoa or the embryos.
| |||||||
| (From Herdman, “Challenger” Report.) | |||||||
|
Fig. 2.—Diagrammatic section of part of Mantle and Test of an Ascidian, showing the formation of a vessel and the structure of the test. | |||||||
| m, | Mantle. | blc, | Bladder cell. | mc, | Mantle cells. | ||
| e, | Ectoderm. | s, s′, | Blood sinus in mantle being drawn out into test. | y, | Septum of vessel. | ||
| tc, | Test cell. | ||||||
| tm, | Matrix. | ||||||
|
|
(From The Cambridge Natural History, vol. vii., “Fishes, &c.” By permission of Macmillan & Co., Ltd.) |
|
Fig. 3.—Section through the surface layer of Test of Ascidia mentula. |
|
bl, Bladder cells; tc, test cell; tk, terminal knobs of vessels; v, vessels of test. |
The outer grey part of the body, which is attached at or near its The Test. posterior end and penetrated by the two apertures, is the “test.” This is a firm gelatinous cuticular secretion upon the outer surface of the ectoderm, which is a layer of flat cells. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the mesoderm. A. Kowalevsky has shown that cells of the mesenchyme of the larva make their way through the ectoderm to the exterior during the metamorphosis, and become the first cells of the young test. Some of the cells in the adult test may, however, be ectodermal in origin (see fig. 2). These test cells may remain as rounded or fusiform or stellate cells embedded in the gelatinous matrix, to which they are constantly adding by secretions on their surfaces; or they may develop vacuoles which become larger and fuse so that each cell has an ovate clear cavity (a bladder cell), surrounded by a delicate film of protoplasm with the nucleus still visible at one point; or they may form pigment granules in the protoplasm; or, lastly, they may deposit carbonate of lime, so that one or several of them together produce a calcareous spicule in the test. Only the unmodified test cells and the bladder cells are found in Ascidia mentula (fig. 3). Calcareous spicules are found chiefly in the Didemnidae amongst compound Ascidians; but pigmented cells may occur in the test of almost all groups of Tunicata. The matrix in which these structures are embedded is usually clear and apparently homogeneous; but in some cases it becomes finely fibrillated, especially in the family Cynthiidae. It is this matrix which contains tunicine. At one point on the left side near the posterior end a tube enters the test, and then splits up into a number of branches, which extend in all directions and finally terminate in rounded enlargements or bulbs, situated chiefly in the outer layer of the test. These tubes are known as the “vessels” of the test, and they contain blood. Each vessel is bounded by a layer of ectoderm cells lined by connective tissue (fig. 4, B), and is divided into two tubes by a septum of connective tissue. The septum does not extend into the terminal bulb, and consequently the two tubes communicate at their ends (fig. 4, A). The vessels are formed by an outgrowth of a blood sinus (derived originally from the blastocoele of the embryo) from the body wall (mantle) into the test, the wall of the sinus being formed by connective tissue and pushing out a covering of ectoderm in front of it (fig. 2, s′). The test is turned inwards at the branchial and atrial apertures to line two funnel-like tubes—the branchial siphon leading to the branchial sac, and the atrial siphon leading to the atrial or peribranchial cavity.
| |
| Fig. 4. | |
| A, | A vessel from the test. |
| B, | Diagrammatic transverse section of a vessel. |
| ec, | Ectoderm. |
| c t, | Connective tissue. |
| s, s′, | The two tubes. |
| y, | Septum. |
| t k, | Terminal bulb. |
The body wall, inside the test and the ectoderm, is formed of a layer (the somatic layer of mesoderm) of connective tissue, enclosing muscle fibres, blood sinuses, and nerves. This layer (the mantle) has very much the shape of the test outside it, but at the two Mantle, Body Wall and Body Cavities. apertures it is drawn out to form the branchial and atrial siphons (fig. 5). In the walls of these siphons the muscle fibres form powerful circular bands, the sphincter muscles. Throughout the rest of the mantle the bands of muscle fibres form a rude irregular network. They are numerous on the right side of the body, and almost totally absent on the left. The muscles are all formed of, very long usiform non-striped fibres. The connective tissue of the mantle is chiefly a clear gelatinous matrix, containing cells of various shapes; it is frequently pigmented, giving brilliant red or yellow colours to the body, and is penetrated by numerous lacunae, in which the blood flows. Inside the mantle, in all parts of the body, except along the ventral edge, there is a cavity—the atrial or peribranchial cavity—which opens to the exterior by the atrial aperture. This cavity is lined by a layer of cells derived originally from the ectoderm[2] and directly continuous with that layer through the atrial aperture (fig. 6); consequently the mantle is covered both externally and internally by ectodermal cells.
| |
|
Fig. 5.—Diagrammatic dissection of A. mentula to show the anatomy. | |
| at, | Atrial aperture. |
| br, | Branchial aperture. |
| a, | Anus. |
| brs, | Branchial sac. |
| dl, | Dorsal lamina. |
| dt, | Dorsal tubercle. |
| end, | Endostyle. |
| h, | Heart. |
| i, | Intestine. |
| m, | Mantle. |
| ng, | Nerve ganglion. |
| œ, | Oesophagus. |
| œa, | Oesephageal aperture. |
| ov, | Ovary. |
| pbr, | Peribranchial cavity. |
| r, | Rectum. |
| st, | Stomach. |
| t, | Test. |
| tn, | Tentacles. |
| vd, | Vas deferens. |
| ngl, | Subneural gland. |
There is no true body cavity or coelom in the mesoderm; and yet the Tunicata are Coelomata in their structure and affinities, although it is very doubtful whether the enterocoele which has been described in the development is really found. In any case the coelom if formed is afterwards suppressed, and in the adult is only represented by the pericardium and its derivatives and the small cavities of the renal and reproductive organs.
The branchial aperture (mouth) leads into the Branchial Sac and Neighbouring Organs. branchial siphon (buccal cavity or stomodaeum), and this opens into the anterior end of a very large cavity (the branchial sac) which extends nearly to the posterior end of the body (see figs. 5 and 6). This branchial sac is an enlarged and modified pharynx, and is therefore properly a part of the alimentary canal. The oesophagus opens from it far back on the dorsal edge (see below). The wall of the branchial sac is pierced by a large number of vertical slits—the stigmata—placed in numerous transverse rows (secondary or subdivided gill-slits.) These slits place the branchial sac in communication with the peribranchial or atrial cavity, which lies outside it (fig. 6). Between the stigmata the wall of the bronchial sac is traversed by blood-vessels, which are arranged in three regular series (fig. 7)—(1) the transverse vessels, which run horizontally round the wall and open at their dorsal and ventral ends into 'large longitudinal vessels, the dorsal and ventral sinuses; (2) the fine longitudinal vessels, which run vertically between adjacent transverse vessels and open into them, and which bound the stigmata.; and (3) the internal longitudinal bars, which run vertically in a plane internal to that of the transverse and fine longitudinal vessels. These bars communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the lumen of the sac in the form of hollow papillae. The edges of the stigmata are richly set with cilia, which drive the water from the branchial sac into the peribranchial cavity, and so cause the currents that flow in through the branchial aperture and out through the atrial.
| ||||
|
(From The Cambridge Natural History, vol. vii., “Fishes, &c.” By permission of Macmillan & Co., Ltd.) | ||||
|
Fig. 6.—Semi-diagrammatic transverse section of Ascidia, passing through the atrial aperture, seen from anterior surface, left side uppermost. | ||||
| At, | Atrial aperture. | mb, | Muscle-bundles. | |
| atl, | Atrial lobe. | ov, | Ovary. | |
| Brs, | Branchial sac. | pbr, | Peribranchial cavity. | |
| cl, | Cloaca. | r, | Rectum. | |
| con, | Connective. | ren, | Renal vesicles. | |
| dbls, | Dorsal blood-sinus. | sg, | Stigmata. | |
| dl, | Dorsal lamina. | sph, | Atrial sphincter. | |
| end, | Endostyle. | t, | Test. | |
| gd, | Genital ducts. | tr, | Transverse vessel. | |
| ii′, | Intestine. | ty, | Typhlosole. | |
| iv, | Interstigmatic vessel. | vbls, | Ventral blood-sinus. | |
| m, | Mantle. | |||
| ||||
| (From Herdman, “Challenger” Report.) | ||||
|
Fig. 7.—A, Part of branchial sac of Ascidia from inside. B, Transverse section of same. | ||||
| tr, | Transverse vessel. | lv, | Fine longitudinal vessels. | |
| cd, | Connecting duct. | p, p′ | Papillae. | |
| hm, | Horizontal membrane. | sg, | Stigmata. | |
| il, | Internal longitudinal bar. | |||
|
(A and B are drawn to different scales.) | ||||
|
|
Fig. 8.—Transverse section of the endostyle of an Ascidian. |
|
br., Branchial sac; end., lips of endostyle; gl., glandular tracts; m.b., muscle bands; pbr., peribranchial cavity; sg., stigma; v.v, ventral vessel. |
Along its ventral edge the wall of the branchial sac is continuous externally with the mantle (fig. 6), while internally it is thickened Endostyle. to form two parallel longitudinal folds bounding a groove, the “endostyle” or ventral furrow (figs. 5, 6,8, end.) corresponding to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. The endoderm cells which line the endostyle are greatly enlarged at the bottom, where they bear very long cilia, and on parts of the sides of the furrow so as to form projecting glandular pads (fig. 8, gl.). It is generally supposed that this organ is a gland for the production of the mucous secretion which is spread round the edges of the branchial sac and catches the food particles in the passing current of water. It has, however, been pointed out that there are comparative few gland cells in the epithelium of the endostyle, and that it is possible that this furrow is merely a ciliated path along which the mucous secretion (produced in part by the subneural gland) is conveyed posteriorly along the ventral edge of the branchial sac. There are sensory bipolar cells in the lateral walls of the endostyle. At its anterior end the Peripharyngeal Bands. edges of the endostyle become continuous with the right and left halves of the posterior of two circular ciliated ridges—the peripharyngeal bands—which run parallel to one another round the front of the branchial sac. The dorsal ends of the posterior peripharyngeal band bend posteriorly Dorsal Lamina. (enclosing the epibranchial roove), and then join to form the anterior end of a fgold which runs along the dorsal edge of the branchial sac as far as the oesophageal aperture. This fold is the dorsal lamina (figs. 5, 6, dl). It probably serves to direct the stream of food particles entangled in a string of mucus from the anterior part of the dorsal lamina to Dorsal Languets. the oesophagus. In many Ascidians this organ, instead of being a continuous membranous fold as in A. mentula, is represented by a series of elongated triangular processes—the dorsal languets—one attached in the dorsal median line opposite to each transverse vessel of the branchial sac. The anterior peripharyngeal band is a complete circular ridge, having no connexion with either the endostyle or the dorsal lamina. In front of it lies the prebranchial zone, which separates the branchial sac behind from the branchial siphon in front. The prebranchial zone is bounded anteriorly by a muscular band—the posterior edge of the sphincter muscle—which bears a circle of long delicate Tentacles. processes, the tentacles (figs. 5, 9, 10, tn). These project inwards at right angles so as to form a network across the entrance to the branchial sac. Each tentacle consists of connective tissue covered with epithelium (endoderm), and contains two or more cavities which are continuous with blood sinuses in Subneural Gland. the mantle. In the dorsal median line near the anterior end of the body, and embedded in the mantle on the ventral surface of the nerve ganglion, there lies a small glandular mass—the subneural gland—which, as Julin has shown (24), there is reason to regard as the homologue of the hypophysis cerebri of the vertebrate brain. Julin and E. van Beneden have suggested that the function of this organ may possibly be renal. The subneural gland, which was first noticed by Hancock, communicates anteriorly, as Ussoff (23) pointed out, by means of a narrow duct with the front of the branchial sac (pharynx). The opening of the duct is enlarged to form a funnel-shaped cavity, which may be folded upon itself, convoluted, or even broken up into a number of smaller Dorsal Tubercle. openings, so as to form a complicated projection, called the dorsal tubercle, situated in the dorsal part of the prebranchial zone. (fig. 9). The dorsal tubercle in A. mentula is somewhat horseshoe-shaped (fig. 10); it varies in form in most Ascidians according to the genus and species, and in some cases in the individual also. The function of the neural gland must still be regarded as doubtful. The secretion is formed by the degeneration and disintegration of cells proliferated from the walls of the duct or its branches, and no concretions are found. The ciliated funnel of the dorsal tubercle is a sense-organ, innervated by a large nerve from the ganglion; it may be a sense-organ for testing the quality of the water entering the bronchial sac.
| |
|
Fig. 9.—Diagrammatic section through anterior dorsal part of A. mentula, showing the relations of the nerve ganglion, subneural gland, &c. | |
| n, | Nerve. |
| n′, | Myelon. |
| pp, | Peripharyngeal band. |
| sgl, | Subneural gland. |
| sgd, | Its duct. |
| t, | Test lining branchial siphon. |
The single elongated ganglion in the median dorsal line of the mantle between the branchial and atrial siphons is the only Nervous System. nerve-centre in A. mentula and most other Tunicata. It is the degenerate remains of the anterior part of the cerebro-spinal nervous system of the tailed larval Ascidian (see below). The posterior or spinal part has entirely disappeared in most Tunicata. It persists, however, in the Appendiculariidae and traces of it are found in some Ascidians (e.g. Clavelina). The ganglion gives off distributor nerves at both ends, which run through the Sense-Organs. mantle to the neighbourhood of the apertures, where they divide and subdivide. The only sense-organs are the pigment spots between the branchial and atrial lobes, the tentacles at the base of the branchial siphon, the dorsal tubercle, and possibly the languets or dorsal lamina. These are all in a lowly developed condition. Nerve-endings have also been found in the endostyle, the peripharyngeal bands and other parts of the wall of the pharynx. The larval Ascidians, on the other hand, have well-developed intracerebral optic and otic sense-organs; and in some of the pelagic Tunicata otocysts and pigment spots or eyes are found in connexion with the ganglion. Atrial tentacles (which may also be sensory) have now been found in a number of the gregarious Cynthiidae and Polystyelidae.
The mouth and the pharynx (branchial sac) have already been Alimentary Canal. described. The remainder of the alimentary canal is a bent tube which in A. mentula and most other Ascidians lies embedded in the mantle on the left side of the body, and projects into the peribranchial cavity. The oesophagus leaves the bronchial sac in the dorsal middle line near the posterior end of the dorsal lamina (see fig. 5, œa). It is a short curved tube which leads ventrally to the large fusiform thick-walled stomach. The intestine emerges from the ventral end of the stomach, and soon turns anteriorly, then dorsally, and then posteriorly so as to form a curve—the intestinal loop—open posteriorly. The intestine now curves anteriorly again, and from this point runs nearly straight forward as the rectum, thus completing a second curve—the rectal loop—open anteriorly (see fig. 5). The wall of the intestine is thickened internally to form the typhlosole, a pad which runs along its entire length. The anus opens into the dorsal part of the peribranchial cavity near to the atrial aperture. The walls of the stomach are glandular; and a system of delicate tubules with dilated ends, which ramifies over the outer wall of the intestine and communicates with the cavity of the stomach by means of a duct, is probably a digestive gland.
|
|
Fig. 10.—Dorsal Tubercle and neighbouring organs of A. mentula.
Lettering as before. |
A mass of large clear vesicles which occupies the rectal loop, and may extend over the adjacent walls of the intestine, is a renal organ Excretory Organs. without a duct. Each vesicle is the modified remains of a part of the primitive coelom or body cavity, and is formed of cells which eliminate nitrogenous waste matters from the blood circulating in the neighbouring blood lacunae and deposit them in the cavity of the vesicle, where they form a concentrically laminated concretion of a yellowish or brown colour. These concretions contain uric acid, and in a large Ascidian are very numerous. The nitrogenous waste products are thus deposited and stored up in the renal vesicles in place of being excreted from the body. In other Ascidians the renal organ may differ from the above in its position and structure; but in no case has it an excretory duct, unless the subneural gland is to be regarded as an additional renal organ.
The heart is an elongated fusiform tube placed on the ventral and posterior, edge of the stomach, in a space (the pericardium) Blood Vascular System and Coelom. which is part of the original coelom or body cavity, the rest of which exists merely in the form of lacunae and of the cavities of the reproductive organs and renal vesicles in the adult Ascidian. The wall of the heart is formed of a layer of epithelio-muscular cells, the inner ends of which are cross-striated; and waves of contraction pass along it from end to end, first for a certain number of beats in one direction and then in the other, so as to reverse the course of circulation periodically. At each end the heart is continued into a vessel (see fig. 11), which is merely a large sinus or lacuna lined with a delicate endothelial layer. The sinus leaving the ventral end of the heart is called the branchio-cardiac vessel,[3] and the heart itself is merely the differentiated posterior part of this sinus and is therefore a ventral vessel. The branchio-cardiac vessel, after giving off a branch which, along with a corresponding branch from the cardio-visceral vessel, goes to the test, runs along the ventral edge of the branchial sac externally to the endostyle, and communicates laterally with the ventral ends of all the transverse vessels of the branchial sac. The sinus leaving the dorsal end of the heart is called the cardio-visceral vessel, and this, after giving off to the test the branch above mentioned, breaks up into a number of sinuses, which ramify over the alimentary canal and the other viscera. These visceral lacunae finally communicate with a third great sinus, the viscero-branchial vessel, which runs forward along the dorsal edge of the branchial sac externally to the dorsal lamina and joins the dorsal ends of all the transverse vessels of the branchial sac. Besides these three chief systems, there are numerous lacunae in all parts of the body, by means of which anastomoses are established between the different currents of blood. All these blood spaces and lacunae are to be regarded as derived from the blastocoele of the embryo, and not, as has been usually supposed, from the coelom (30). When Course of Circulation. the heart contracts ventro-dorsally the course of the circulation is as follows: the blood which is flowing through the vessels of the branchial sac is collected in an oxygenated condition in the branchio-cardiac vessel, and, after receiving a stream of blood from the test, enters the heart (ht). It is then propelled from the dorsal end of the heart into the cardio-visceral vessels, and so reaches the test and digestive and other organs; then, after circulating in the visceral lacunae, it passes into the branchio-visceral vessel in an impure condition, and is distributed to the branchial vessels (fig. 11, da) to be purified again. When the heart on the other hand contracts dorso-ventrally, this course of the circulation is reversed. As the test receives a branch from each end of the heart, it follows that it has afferent and efferent vessels whichever way the blood is flowing. In some Ascidians the vessels in the test become very numerous and their end branches terminate in swollen bulbs close under the outer surface of the test. In this way an accessory respirator organ is probably formed in the superficial layer of the test. The blood corpuscles are chiefly colourless and amoeboid; but in most if not all Ascidians there are also some pigmented corpuscles in the blood. These are generally of an orange or reddish brown tint, but may be opaque white, dark indigo-blue, or even of other colours. Precisely similarly pigmented cells are found throughout the connective tissue of the mantle and other parts of the body.
| ||||||||||||||
|
Fig. 11.—Diagram of the Blood Circulation in an Ascidian. The test is solid black. | ||||||||||||||
| ||||||||||||||
A. mentula is hermaphrodite, and the reproductive organs lie, with the alimentary canal, on the left side of the body. The ovary Reproductive Organs. is a ramified gland which occupies the greater part of the intestinal loop (see fig. 5). It contains a cavity which, along with the cavities of the testis, is derived from a part of the original coelom, and the ova are formed from its walls and fall when mature into the cavity. The oviduct is continuous with the cavity of the ovary and leads forwards alongside the rectum. finally opening near the anus into the peribranchial cavity. The testis is composed of a great number of delicate branched tubules, which ramify over the ovary and the adjacent parts of the intestinal wall. Those tubules terminate in ovate swellings. Near the commencement of the rectum the larger tubules unite to form the vas deferens, a tube of considerable size, which runs forwards alongside the rectum, and, like the oviduct, terminates by opening into the peribranchial cavity close to the anus. The lumen of the tubules of the testis, like the cavity of the ovary, is a part of the original coelom, and the spermatozoa are formed from the cells lining the wall. In some Ascidians reproductive organs are present on both sides of the body, and in others (Polycarpa) there are many complete sets of both male and female systems, attached to the inner surface of the mantle on both sides of the body and projecting into the peribranchial cavity.[4]
Embryology[5] and Life-History
We owe to W. E. Castle (1896) the most complete account which has yet been given of the early stages of development in an Ascidian. His careful study of the cell lineage in Ciona has made it clear that some of the conflicting statements of his predecessors arose from incorrect orientation of the embryos. One of the most important of his conclusions is that the mesoderm of Ascidians, and probably that of the archaic Vertebrates, is derived from both primary layers, ectoderm and endoderm. Further, he finds that Ciona produces both ova and spermatozoa at the same time, but self-fertilization very rarely occurs. The eggs are laid just before dawn, and the larva is hatched during the following night. The test cells adhering to the young homogeneous test have, it is now well known, no connexion with the cells found later in the adult test. The larvae are free-swimming for from one to several days. They avoid the light. The spermatozoon enters at the ventral hemisphere, and that point determines the median plane and the posterior end of the embryo. The ventral is the animal pole. The cleavage is from the beginning bilateral. The first cleavage plane is vertical, and separates the right and left halves of the embryo. The four smaller dorsal cells with yolk give rise to the endodermal hemisphere; the four larger, more protoplasmic, cells form the ventral ectodermal hemisphere. The cells of the latter hemisphere divide more rapidly, and form the future aboral surface. When the dorsal hemisphere has twenty-two cells the ventral has fifty-four. The gastrulation is a combination of epiboly and invagination. The ventral ectoderm grows over, so as to envelop the dorsal hemisphere, while the latter sinks down and becomes saucer-shaped. In the centre of the dorsal surface ten cells form the future endoderm. Round these comes a ring of cells, the chordamesenchyme ring, from which the notochord and mesenchyme arise. Outside this ring is a row of cells, the neuro-muscular ring. The more anterior of these cells form the medullary plate, the more posterior the longitudinal musculature of the larva. The remainder of the cells (in the 112-cell stage) form ectoderm. By growth at the anterior end the blastopore gets pushed posteriorly, and the anterior chorda cells are covered up, and come to lie in the dorsal wall of the archenteron, sixteen cells in two rows, one over the other. The blastopore closes in the posterior part of the dorsal surface. In front of it is the medullary plate, with a continuation backwards at the sides of the blastopore. This region forms the trunk of the larva, the part posterior to it being drawn out to form the tail. The chorda cells pass back into the tail, while the mesenchyme cells shift forwards into the trunk. The muscle cells, derived from the neuro-muscular ring, lie behind the blastopore, and form the muscles of the tail. The closure of the medullary canal takes place from the blastopore forwards, and then the nerve cord is grown over by ectoderm. After closure of the blastopore the mesenchyme cells lie as lateral masses in the trunk; later they become the blood corpuscles and the mantle cells, &c.
Castle also discusses some important theoretical questions. He points out that, in Ciona at least, the chorda-mesenchyme ring takes part along with endoderm in the primary invagination, and so belongs to the primary endoderm; while the rest of the mesoderm, the muscle cells of the neuro muscular ring, are carried in by a secondary invagination, and belong to the outer layer-of the young gastrula, or primary ectoderm. He considers that the chorda must be regarded as a mesodermal organ. He agrees with former observers in seeing no trace of enterocoele formation, and he doubts whether any Chordata are Enterocoela. He does not believe in distinguishing those Metazoa with a mesoderm from those with a “mesenchyme.” He considers that embryology gives no support to the Annelid hypothesis as to the origin of Chordates.
A long-continued discussion as to the origin, nature and fate of certain cells, the “testa-zellen,” which make their appearance between the young embryo and its follicle (fig. 12), has ended in practical agreement that these small cells are derived from the follicle-cells, and have nothing to do with the test. In Salpa, however, certain follicle-cells enter the embryo, and perform important functions in guiding the development for a time.
|
|
(After Pizon.) |
|
Fig. 12.—Portion of Mature Ovum of Ascidian, showing F, follicle, and f, r, “test-cells.” |
In most Ascidians the eggs are fertilized in the peribranchial cavity, and undergo most of their development before leaving the Embryology. parent; in some cases, however, the eggs are laid, and fertilization takes place in the surrounding water. The segmentation is complete and regular (fig. 13, A) and results in the formation of a spherical blastula, which then undergoes invagination (fig. 13, B). The embryo elongates, and the blastopore or invagination opening comes to be placed on the dorsal edge near the posterior end (fig. 15, C). The hypoblast cells lining the archenteron are columnar in form, while the epiblast cells are more cubical (fig. 13, B, C, D). The dorsal surface of the embryo now becomes flattened and then depressed to form a longitudinal groove, extending forwards from the blastopore to near the front of the body, This “medullary groove” now becomes converted into a closed canal by its side walls growing up, arching over, and coalescing in the median dorsal line (fig. 13, D). This union of the laminae dorsales to form the neural canal commences at the posterior end behind the blastopore and gradually extends forwards. Consequently the blastopore comes to open into the posterior end of the neural canal (fig. 13, D), while the anterior end of that cavity remains open to the exterior. In this way the archenteron communicates indirectly with the exterior. The short canal leading from the neural canal to the archenteron is known as the neurenteric canal (fig. 13, D, nec). Previous to this stage some of the hypoblast cells at the front edge of the blastopore and forming part of the dorsal wall of the archenteron (fig. 13, C, ch) have become separated off, and then arranged to form an elongated band, two cells wide, underlying the posterior half of the neural canal (fig. 13, D, E, ch). This is the origin of the notochord. Outgrowths from the sides of the archenteron give rise to laterally placed masses of cells, which are the origin of the mesoblast. These masses show no trace of metameric segmentation. The cavities (reproductive and renal vesicles) which are formed later in the mesoblast represent the coelom. Consequently the body cavity of the Tunicata is a modified form of enterocoele. The anterior part of the embryo, in front of the notochord, now becomes enlarged to form the trunk, while the posterior part elongates to form the tail (fig. 13, E). In the trunk the anterior part of the archenteron dilates to form the mesenteron, the greater part of which becomes the branchial sac; at the same time the anterior part of the neural canal enlarges to form the cerebral vesicle, and the opening to the exterior at the front end of the canal now closes. In the tail part of the embryo the neural canal remains as a narrow tube, while the remains of the wall of the archenteron-the dorsal part of which becomes the notochord—are converted into lateral muscle bands (fig. 13, G) and a ventral cord of cells, which eventually breaks up to form blood corpuscles. As the tail grows longer, it becomes bent round the trunk of the embryo inside the egg-membrane. About this period the epiblast cells begin to form the test as a cuticular deposit upon their outer surface. The test is at first devoid of cells and forms a delicate gelatinous investment, but it shortly afterwards becomes cellular by the migration into it of test cells formed by proliferation from the epiblast.[6]
| ||||||||||||||||||||||||||||||||||
| (After Kowalevsky.) | ||||||||||||||||||||||||||||||||||
| Fig. 13.—Stages in the Embryology of a Simple Ascidian. | ||||||||||||||||||||||||||||||||||
|
A to F, Longitudinal vertical sections of embryos, all placed with the dorsal surface uppermost and the anterior end at the right. A, Early blastula stage, during segmentation. B, Early gastrula stage. C, Stage after gastrula, showing commencement of notochord. D, Later stage, showing formation of notochord and of neural canal. E, Embryo showing body and tail and completely formed neural canal. F, Larva just hatched; end of tail cut off. G, Transverse section of tail of larva. | ||||||||||||||||||||||||||||||||||
|
The embryo is hatched about two or three days after fertilization, in the form of a tadpole-like larva, which swims actively through Larval Stage. the sea by vibrating its long tail. The anterior end of the body is provided with three adhering papillae (fig. 13, F, adp.) in the form of epibiastic thickenings. In the free-swimming tailed larva the nervous system, formed from the walls of the neural canal, becomes considerably differentiated. The anterior part of the cerebral vesicle remains thin-walled (fig. 13, F), and two unpaired sense-organs develop from its wall and project into the cavity. These are a dorsally and posteriorly placed optic organ, provided with retina, pigment layer, lens and cornea, and a ventrally placed auditory organ, consisting of a large spherical partially pigmented otolith, attached by delicate hair-like processes to the summit of a hollow crista acoustica (fig. 13, F, au). The posterior part of the cerebral vesicle thickens to form a solid ganglionic mass traversed by a narrow central canal: this becomes the ganglion of the adult Ascidian. The wall of the neural canal behind the cerebral vesicle becomes differentiated into an anterior thicker region, placed in the posterior part of the trunk and having a superficial layer of nerve fibres, and a posterior narrower part which traverses the tail, lying on the dorsal surface of the notochord, and gives off several pairs of nerves to the muscles of the tail. Just in front of the anterior end of the nervous system a dorsal involution of the epiblast breaks through into the upturned anterior end of the mesenteron and thus forms the mouth opening. Along the ventral edge of the mesenteron, which becomes the branchial sac, the endostyle is formed as a narrow groove with thickened side walls. It probably corresponds to the median portion of the thyroid body of Vertebrata. A curved outgrowth from the posterior end of the mesenteron forms the alimentary canal (oesophagus, stomach and intestine), which at first ends blindly. An anus is formed later by the intestine opening into the left of two lateral epiblastic involutions (the atria), which rapidly become larger and fuse dorsally to form the peribranchial cavity. Outgrowths from the wall of the branchial sac meet these epiblastic involutions and fuse with them to give rise to the first formed pair of stigmata, which thus come to open into the peribranchial cavity; and these alone correspond to the gill clefts of Amphioxus and the Vertebrata.
|
|
Fig. 14.—Sketches of Ascidian Larvae. |
Fig. 14 shows a few characteristic forms of Ascidian “tadpoles,” or free-swimming larvae. A and S are typical simple Ascidians; M is the aberrant tailless form found in some Molgulidae; and C is the larva of a typical compound Ascidian.
After a short free-swimming existence the fully developed tailed larva fixes itself by its anterior adhering papillae to some foreign Metamorphosis to Adult Form. object, and then undergoes a remarkable series of retrogressive changes, which convert it into the, adult Ascidian. The tail atrophies, until nothing is left but some fatty cells in the posterior part of the trunk. The adhering papillae disappear and are replaced functionally by a growth of the test over neighbouring objects. The nervous system with its sense organs atrophies until it is reduced to the single small ganglion, placed on the dorsal edge of the pharynx, and a slight nerve cord running for some distance posteriorly (van Beneden and Julin). Changes in the shape of the body and a further growth and differentiation of the branchial sac, peribranchial cavity and other organs now produce gradually the structure found in the adult Ascidian.
The most important points in connexion with this process of development and metamorphosis are the following: (1) In the Ascidian embryo all the more important organs (e.g. notochord, neural canal, archenteron) are formed in essentially the same manner as they are in Amphioxus and other Chordata. (2) The free-swimming tailed larva possesses the essential characters of the Chordata, inasmuch as it has a longitudinal skeletal axis (the notochord) separating a dorsally placed nervous system (the neural canal) from a ventral alimentary canal (the archenteron); and therefore during this period of its life-history the animal belongs to the Chordata. (3) The Chordate larva is more highly organized than the adult Ascidian, and therefore the changes by which the latter is produced from the former may be regarded as a process of degeneration (31). The important conclusion drawn from all this is that the Tunicata are the degenerate descendants of a group of primitive Chordata (see below).
| ||||||||||||||||
|
(From The Cambridge Natural History, vol. vii., “Fishes, &c.” By permission of | ||||||||||||||||
|
Fig. 15.—Metamorphosis of an Ascidian (modified from Kowalevsky and others). | ||||||||||||||||
|
A, Free-swimming tailed larva. B, The metamorphosis—larva attached. C, Tail and nervous system of larva degenerating. D, Further degeneration and metamorphosis of larva into E, the young fixed Ascidian. | ||||||||||||||||
|
|
|
(From The Cambridge Natural History, vol. vii., “Fishes, &c.” By permission of |
| Fig. 16.—Sketch of the chief kinds of Tunicata found in the sea. |
Classification and Characters of Groups
Order I.—Larvacea
|
| (After Fol.) |
|
Fig. 17.—Oikopleura cophocerca in “House,” seen from right side, magnified. The arrows indicate the course of the water. x, Lateral reticulated parts of “House.” |
Free-swimming pelagic forms provided with a large locomotory appendage (the tail), in which there is a skeletal axis (the urochord). Characters of Larvacea. A relatively large test (the “house”) is formed with great rapidity as a secretion from the ectoderm; it is merely a temporary structure, which is cast off and replaced by another. The branchial sac is simply an enlarged pharynx with two ventral ciliated openings (stigmata) leading to the exterior. There is no separate peribranchial cavity. The nervous system consists of a large dorsally placed ganglion and a long nerve cord, which stretches backwards over the alimentary canal to reach the tail, along which it runs on the left side of the urochord. The anus opens ventrally on the surface of the body in front of the stigmata. No reproduction by gemmation or metamorphosis is known in the life-history.
This is one of the most interesting groups (fig. 16) of the Structure of Appendicularis. Tunicata, as it shows more completely than any of the rest the characters of the original ancestral forms. It has undergone little or no degeneration, and consequently corresponds more nearly to the tailed-larval condition than to the adult forms of the other groups. The order includes a single family, the Appendiculariidae, all the members of which are minute and free-swimming. They occur on the surface of the sea in most parts of the world. They possess the power to form with great rapidity an enormously large investing gelatinous layer (fig. 11), which corresponds to the test of other groups. This was first described by von Mertens and by him named “Haus.” It is only loosely attached to the body and is frequently thrown off soon after its formation and again reformed. H. Lohmann has made a careful study of the mode of formation of this “house” from certain large ectoderm cells, the “oikoplasts,” and he considers that it probably fulfils the following functions: Its complicated apparatus of passages with partial septa form a finely perforated network, through which a relatively large volume of water is strained so as to entrap microscopic food particles; it helps in locomotion by its hydrostatic effect, and it is also a protection to the animal, which may escape from enemies by throwing off the house, which is many times its own size. The tail in the Appendiculariidae is attached to the ventral surface of the body (fig. 18), and usually points more or less anteriorly. The supposed traces of vertebration in the muscle bands and the nerve cord are probably artifacts, and do not indicate true metameric segmentation. Near the base of the tail there is a distinct elongated ganglion (fig. 18, ng′). The anterior (cerebral) ganglion has connected with it an otocyst, a pigment spot, and a tubular process opening into the branchial sac and representing the dorsal tubercle and associated parts of an ordinary Ascidian. The branchial aperture or mouth leads into the branchial sac or pharynx. There are no tentacles. The endostyle is short. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly. The wall of the bronchial sac has only two ciliated apertures (fig. 19). They are homologous with the primary stigmata of the typical Ascidians and the gill clefts of vertebrates. They are placed far back on the ventral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (fig. 18, at). These tubes correspond to the right and left atrial involutions which, in an ordinary Ascidian, fuse to form the peribranchial cavity. The heart, according to Lankester, is formed of two cells, which are placed at the opposite ends and connected by delicate contractile protoplasmic fibrils. The large ovary and testis are placed at the posterior end of the body. The remainder of the structural details can be made out from figs. 18 and 19.
| ||||||||||||||||||||||||||||||||||||||||||||||||||
| Fig. 18.—Semi-diagrammatic view of Appendicularia from the
right. | ||||||||||||||||||||||||||||||||||||||||||||||||||
|
| ||||||||||||||||||||
|
Fig. 19.—Transverse Section of Oikopleura; anterior part of body and tail. | ||||||||||||||||||||
|
The family Appendiculariidae comprises amongst others the following genera: Oikopleura (Mertens), and Appendicularia (Cham.), in both of which the body is short and compact and the tail relatively long, while the endostyle is straight; Megalocercus (Chun) containing M. abyssorum, a huge deep-sea form from the Mediterranean (30 mm. long); Fritillaria (Quoy and Gaimard), in which the body is long and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, and an ectodermal hood is formed over the front of the body; and Kowalevskia (Fol), a remarkable form described by Fol (14), in which the heart and endostyle are said to be absent, while the branchial sac is provided with four rows of ciliated tooth-like processes.
Order II.—Thaliacea
Free-swimming pelagic forms which may be either simple or compound, and the adult of which is never provided with a tail or Thaliacea. a notochord. The test is permanent and may be either well developed or very slight. The musculature of the mantle is in the form of more or less complete circular bands, by the contraction of which locomotion is effected. The branchial sac has either two large or many small apertures, leading to a single peribranchial cavity, into which the anus opens. Blastogenesis takes place from a ventral endostylar stolon. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Thaliacea comprises two groups Cyclomyaria and Hemimyaria.
Sub-order 1.—Cyclomyaria.
Free-swimming pelagic forms which exhibit alternation of generations in their life-history but never form permanent colonies. The Characters of Cyclomyaria. body is cask-shaped, with the bronchial and atrial apertures at the opposite ends. The test is more or less well developed. The mantle has its musculature in the form of circular bands surrounding the body. The branchial sac is fairly large, occupying the anterior half or more of the body. Stigmata are usually present in its posterior part only. The peribranchial cavity is mainly posterior to the branchial sac. The alimentary canal is placed ventrally close to the posterior end of the branchial sac. Hermaphrodite reproductive organs are placed ventrally near the intestine.
This group forms one family, the Doliolidae, including three genera, Doliolum (Quoy and Gaimard), Dolchinia (Korotneff) and Anchinia (C. Vogt).
| ||||||||||||||||
|
Fig. 20.—Doliolum denticulatum, sexual generation, from the left side. Lettering as for fig. 18. | ||||||||||||||||
|
Doliolum, of which about a dozen species are known from various seas, has a cask-shaped body, usually from 1 to 2 cm. in length. The terminal bronchial and atrial apertures (fig. 20) are lobed and the lobes are provided with sense organs. The test is very slightly developed and contains no cells. The mantle has eight or nine circular muscle bands surrounding the body. The most anterior and posterior of these form the bronchial and atrial sphincters. The wide branchial and atrial apertures lead into large branchial and peribranchial cavities, separated by the posterior wall of the branchial sac, which is pierced by stigmata; consequently there is a free passage for the water through the body along its long axis, and the animal swims by contracting its ring-like muscle bands, so as to force out the contained water posteriorly. Stigmata may also be found on the lateral walls of the branchial sac, and in that case there are corresponding anteriorly directed diverticula of the peribranchial cavity. There is a distinct endostyle on the ventral edge of the branchial sac and a peripharyngeal band surrounding its anterior end, but there is no representative of the dorsal lamina on its dorsal edge. The oesophagus commences rather on the ventral edge of the posterior end of the branchial sac, and runs backwards to open into the stomach, which is followed by a curved intestine opening into the peribranchial cavity. The alimentary canal as a whole is to the right of the middle line. The hermaphrodite reproductive organs are to the left of the middle line alongside the alimentary canal. They open into the peribranchial cavity. The ovary is nearly spherical, while the testis is elongated, and may be continued anteriorily for a long distance. The heart is placed in the middle line ventrally, between the posterior end of the endostyle and the oesophageal aperture. The nerve ganglion lies about the middle of the dorsal edge of the body, and gives off many nerves. Under it is placed the subneural gland, the duct of which runs forward and opens into the anterior end of the branchial sac by a simple aperture, surrounded by the spirally twisted dorsal end of the peripharyngeal band (fig. 20., dt).
The ova of the sexual generation produce tailed larvae; these develop into forms known as “nurses,” which are asexual, and are Development of Doliolum. characterized by the possession of nine muscle bands, an auditory sac on the left side of the body, a ventrally-placed stolon near the heart, upon which buds are produced and a dorsal outgrowth near the posterior end of the body. The nurse after producing the buds becomes a degenerate form with very wide muscle bands. The buds give rise eventually to the sexual generation, which is polymorphic, having three distinct forms, in two of which the reproductive organs remain undeveloped. The buds while still very young migrate from their place of origin on the stolon, divide by fission, and become attached to the dorsal outgrowth of the body of the nurse, where they develop. The three forms produced are as follows. (1) Nutritive forms (trophozooids), which remain permanently attached to the nurse and serve to provide it with food; they have the body elongated dorso-ventrally, and the musculature is very slightly developed. (2) Foster forms (phorozooids), which, like the preceding, do not become sexually mature, but, unlike them, are set free as cask-shaped bodies with eight muscle bands and a ventral outgrowth, which is formed of the stalk by which the body was formerly united to the nurse. On this outgrowth the (3) forms (gonozooids) which become sexually mature are attached while still young buds, and after the foster forms are set free these reproductive forms gradually attain their complete development and are eventually set free and lose all trace of their connexion with the foster forms. They resemble the foster forms in having a cask-shaped body with eight muscle bands, but differ in having no outgrowth or process, and in having the reproductive organs fully developed.[7]
Anchinia, of which only one species is known, A. rubra, from the Mediterranean, has the sexual forms permanently attached to Anchinia. portions of the dorsal outgrowth from the body of the unknown nurse. The body is elongated dorso-ventrally. The test is well developed and contains branched cells. The musculature is not so well developed as in Doliolum. There are two circular bands at the anterior end and two at the posterior, and two on the middle of the body. The stigmata are confined to the obliquely placed posterior end of the branchial sac. The alimentary canal forms a U-shaped curve. The reproductive organs are placed on the right side of the body. The life-history is still imperfectly known. As in the case of Doliolum the sexual generation is polymorphic, and has three forms, two of which remain in a rudimentary condition so far as the reproductive organs are concerned. In Anchinia, however, the three forms do not occur together on one stolon or outgrowth, but are produced successively, the reproductive forms of the sexual generation being independent of the “foster forms” (see Barrois, 27).
Sub-order 2.—Hemimyaria.
Free-swimming pelagic forms which exhibit alternation of generations in their life-history and in the sexual condition form colonies. Characters of Hemimyaria. The body is more or less fusiform, with the long axis antero-posterior, and the branchial and atrial apertures nearly terminal. The test is well developed. The musculature of the mantle is in the form of a series of transversely-running bands, which do not form complete independent rings as in the Cyclomyaria. These transverse muscles are probably to be regarded as branchial and atrial sphincters which have spread over the body. The branchial and peribranchial cavities form a continuous space in the interior of the body, opening externally by the branchial and atrial apertures, and traversed obliquely from the dorsal and anterior end to the ventral and posterior by a long narrow vascular band, which represents the dorsal lamina, the dorsal blood-vessel, and the neighbouring part of the dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary canal is placed ventrally. It may either be stretched out (ortho-enteric) so as to extend for some distance anteriorly, or—as is more usual—be concentrated (caryo-enteric) to form along with the reproductive organs a rounded opaque mass near the posterior end of the body known as the visceral mass or “nucleus.” The embryonic development is direct, no tailed larva being formed.
|
|
| A | B |
| Fig. 21.—Salpa runcinata-fusiformis. | |
|
A, Aggregated form: em, Embryo; gem, Gemmiparous stolon; m, Mantle; visc, Visceral mass (nucleus). B, Solitary form: 1-9, Muscle bands. Lettering as before. | |
| |
|
Fig. 22.—Posterior part of solitary form of Salpa democratica-mucronata, showing a chain of embryos nearly ready to be set free. | |
| gem, | Young aggregated Salpae forming the chain. |
| st, | Stolon. |
| m, | Muscle band of the mantle. |
This sub-order contains one family, the Salpidae, including the single Salpidae. genus Salpa (Forskål), which, however, may be divided into two well-marked groups of species—(1) those, such as S. pinnata, in which the alimentary canal is stretched out along the ventral surface of the body, and (2) those, such as S. fusiformis (fig. 21, A), in which the alimentary canal forms a compact globular mass, the “nucleus,” near the posterior end of the body. About fifteen species altogether are known; they are all pelagic forms and are found in nearly all seas. Each species occurs in two forms—the solitary asexual (proles solitaria) and the aggregated sexual (proles gregaria)—which are usually quite unlike one another. The solitary form (fig. 21, B) gives rise by internal gemmation to a complex tubular stolon, which contains processes from all the more important organs of the parent body and which becomes segmented into a series of buds or embryos. As the stolon elongates, the embryos near the free end which have become advanced in their development are set free in groups, which remain attached together by processes of the test, each enclosing a diverticulum from the mantle so as to form “chains” (fig. 22). Each member of the chain is a Salpa of the sexual or aggregated form, and when mature may—either still attached to its neighbours or separated from them (fig. 21, A)—produce one or several embryos, which develop into the solitary Salpa. Thus the two forms alternate regularly.
| ||||||||||||
|
Fig. 23.—Semi-diagrammatic representation of Salpa from left side. Lettering as before. | ||||||||||||
|
The more important points in the structure of a typical Salpa are
shown in fig. 23. The branchial and atrial apertures are at opposite
Structure of Salpa.
ends of the body, and each leads into a large cavity,
the branchial and peribranchial sacs, which are in free
communication at the sides of the obliquely-running
dorsal lamina or “gill.” The test is well developed and adheres
closely to the surface of the mantle. The muscle bands of the
mantle do not completely encircle the body. They are present
dorsally and laterally, but the majority do not reach the ventral
surface. In many cases neighbouring bands join in the median
dorsal line (fig. 21). The anterior end of the dorsal lamina is
prolonged to form a prominent tentacular organ, the languet, projecting
into the branchial sac. The nerve ganglion (which represents the
ganglion of the Ascidian along with the subneural gland), dorsal
lamina, peripharyngeal bands and endostyle, are placed in their usual positions; but in place of any distinct subneural gland there
are two lateral neural glandular masses first described by Metcalf.
These have no connexion with the ciliated funnel, but open by lateral
ducts into the branchial cavity. Median and lateral eyes are also
found in connexion with the ganglion. The large spaces at the sides
of the dorsal lamina (often called the gill or branchia of Salpa), by
means of which the cavity of the branchial sac is placed in free communication
with the peribranchial cavity, are to be regarded as
gigantic stigmata formed by the suppression of the lateral walls of
the branchial sac. Fig. 23 represents an aggregated or sexual Salpa
which was once a member of a chain, since it shows a testis and a
developing embryo. The ova (always few in number, usually only
one) appear at a very early period in the developing chain Salpa,
while it is still a part of the gemmiparous stolon in the body of the
solitary Salpa. This gave rise to the view put forward by Brooks
(25), that the ovary really belongs to the solitary Salpa, which is
therefore a female producing a series of males by asexual gemmation,
and depositing in each of these an ovum, which will afterwards, when
fertilized, develop in the body of the male into a solitary or female
Salpa. This idea would of course entirely destroy the view that
Salpa is an example of alternation of generations. The sexual or
chain Salpa, although really hermaphrodite, is always protogynous;
i.e. the female elements or ova are produced at an earlier period
than the male organ or testis. This prevents self-fertilization.
The ovum is fertilized by the spermatozoa of an older Salpa
Development
of Salpa.
belonging to another chain, and the embryo is far
advanced in its development before the testis is formed.
Follicular cells, known as kalymmocytes, migrate into
the ovum and for a time play an important part in moulding the
development and nourishing the blastomeres. At an early period
in its development a part of the embryo becomes separated off,
along with a part of the wall of the cavity in which it lies, to form
the “placenta,” in which the embryonic and the maternal blood
streams circulate in close proximity (or actually coalesce during one
period) and so allow of the passage of nutriment to the developing
embryo. At a somewhat later stage a number of cells placed at the
posterior end of the body alongside the future nucleus become filled
with oil-globules to form a mass of nutrient material—the
elaeoblast—which is used up later on in the development. Many
suggestions have been made as to the homology of the elaeoblast. The
most probable is that it is the disappearing rudiment of the tail
found in the larval condition of most Ascidians.
Addendum.
The family Octacnemidae includes the single remarkable genus Octacnemus, found during the “Challenger” expedition, and first described by Moseley (29). It is now known in both a solitary and an aggregated form, and was regarded by Herdman as a deep-sea representative of the pelagic Octacnemidae. Salpidae, possibly fixed; or, better, as related to the primitive fixed forms from which Salpidae have been derived. Metcalf, however, has shown that the aggregated form of O. patagoniensis, which he has described, is more nearly related to the Clavelinidae amongst Ascidiacea. The body is somewhat discoid, with its margin prolonged to form eight tapering processes (fig. 24), on to which the muscle bands of the mantle are continued. The alimentary canal forms a compact nucleus (fig. 24, A); the endostyle is very short; and the dorsal lamina is also reduced. The reproduction and life history are entirely unknown. Octacnemus bythius was found by the “Challenger” expedition in the South Pacific at depths of 1070 and 2160 fathoms, and Metcalf has since described a new species, O. patagoniensis from 1050 fathoms off the Patagonian coast, in which there is an aggregated form (fig. 24, B) consisting of individuals united by a stolon composed of test and body-walls.
|
| Fig. 24.—Octacnemus.
A, Solitary form (after Herdman). B, Aggregated form (after Metcalf). |
| a, | Anus. | m, | Mouth. | |
| At, | Atrial aperture. | œ, | Oesophagus. | |
| br.s, | Branchial sac. | p.br, | Peribranchial cavity. | |
| g.s, | Gill slit. | st, | Stolon. |
Order III.—Ascidiacea
Fixed or free-swimming simple or compound Ascidians which in
the adult are never provided with a tail and have no trace of a
notochord. The free-swimming forms are colonies, the
simple Ascidians being always fixed. The test is permanent
and well developed; as a rule it increases with the age of the
Ascidiacea.
individual. The branchial sac is large and well developed. Its
walls are perforated by numerous slits (stigmata) opening into
the peribranchial cavity, which communicates with the exterior by
the atrial aperture. Many of the forms reproduce by geminmation,
and in most of them the sexually-produced embryo develops into a
tailed larva.
The Ascidiacea includes three groups—the simple Ascidians, the compound Ascidians and the free-swimming colonial Pyrosoma.
Sub-Order I.—Ascidiae simplices.
Fixed Ascidians which are solitary and very rarely reproduce by gemmation; if colonies are formed, the members are not buried in a common investing mass, but each has a distinct test of its own. No strict line of demarcation can be drawn between the simple and the compound Ascidians, and Simple Ascidians. one of the families of the former group, the Clavelinidae (the social Ascidians), forms a transition from the typical simple forms, which never reproduce by gemmation, to the compound forms, which always do. The Ascidiae Simplices may be divided into the following families:—
Family I., Clavelinidae.—Simple Ascidians which reproduce by gemmation to form small colonies in which each ascidiozooid has a distinct test, but all are connected by a common blood system, and by prolongations of “epicardiac tubes” from the branchial sacs. Buds formed on stolons which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm and endoderm of the ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. This family contains, amongst others, the following three genera: Ecteinascidia (Herdman), with internal longitudinal bars in branchial sac; Clavelina (Savigny), with intestine extending behind branchial sac; and Perophora (Wiegmann), with intestine alongside branchial sac.
Family II., Ascidiidae.—Solitary fixed Ascidians with gelatinous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed. Branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple. This family is divided into three sections:—
Sub-family 1, Hypobythinae.—Branchial sac with no internal longitudinal bars. One genus, Hypobythius (Moseley).
Sub-family 2, Ascidinae.—Stigmata straight. Many genera, of which the following are the more important: Ciona (Fleming), dorsal languets present; Ascidia (Linnaeus, = Phallusia, Savigny), dorsal lamina present (see figs. 1 to 10); Rhodosoma (Ehrenberg), anterior part of test modified to form operculum; Abyssascidia (Herdman), intestine on right side of branchial sac.
Sub-family 3, Corellinae.—Stigmata curved. Three chief genera: Corella (Alder and Hancock), test gelatinous, body sessile; Corynascidia (Herdman), test gelatinous, body pedunculated; Chelyosoma (Brod. and Sow.), test modified into horny plates.
Family III., Cynthiidae.—Solitary fixed Ascidians, usually with leathery test; branchial and atrial apertures both four-lobed. Branchial sac longitudinally folded (fig. 26); stigmata straight; tentacles simple or compound. This family is divided into three sections:—
Sub-family 1, Styelinae.—Not more than four folds on each side of branchial sac (fig. 26, S) tentacles simple. The more important genera are: Styela (Macleay), stigmata normal, and Bathyoncus (Herdman), stigmata absent or modified.
|
| ||||||||||||||||
| (After Herdman, “Challenger” Report.) | |||||||||||||||||
| Fig. 25.—Culeolus willemoesi. | |||||||||||||||||
| A, Entire body, natural size. | B, Part of branchial sac magnified. | ||||||||||||||||
|
| ||||||||||||||||
Sub-family 2, Cynthinae.—More than eight folds in branchial sac (fig. 26, C); tentacles compound; body sessile. The chief genus is Cynthia (Savigny), with a large number of species.
Sub-family 3, Bolteninae.—More than eight folds in branchial sac; tentacles compound; body pedunculated (fig. 25, A). The chief genera are: Boltenia (Savigny), branchial aperture four-lobed, stigmata normal; and Culeolus (Herdman), branchial aperture with less than four lobes, stigmata absent or modified (fig. 25, B). This last is a deep-sea genus discovered by the “Challenger” expedition (see 17).
Family IV., Molgulidae.—Solitary Ascidians, sometimes not fixed; branchial aperture six-lobed, atrial four-lobed. Test usually incrusted with sand. Branchial sac longitudinally folded; stigmata more or less curved, usually arranged in spirals; tentacles compound. The chief genera are: Molgula (Forbes), with distinct folds in the branchial sac, and Eugyra (Ald. and Hanc.), with no distinct folds, but merely broad internal longitudinal bars in the branchial sac. In some of the Molgulidae (genus Anurella, Lacaze-Duthiers, 20) the embryo (fig. 14, M) does not become converted into a tailed larva, the development being direct, without metamorphosis. The embryo when hatched assumes gradually the adult structure, and never shows the features characteristic of larval Ascidians, such as the urochord and the median sense-organs. Bourne has described an aberrant Molgulid, Oligotrema, from the Loyalty Islands, with a reduced branchial sac and enlarged pinnate muscular branchial lobes, apparently used for catching food!
|
|
Fig. 26.—Diagrams showing Transverse Sections of Typical Branchial Sacs. |
|
A, Unfolded type. S, Styela, with four folds on each side. |
| D.L., Dorsal lamina; End, endostyle; I, II, &c., folds. |
|
|
Fig. 27.—Types of Stomach amongst Compound Ascidians. P, Plain.F, Folded.A, Areolated. i, intestine;œ, oesophagus;st, stomach. |
Figs. 26 and 27 illustrate some details of structure of branchial sac and of stomach in various simple and compound Ascidians, which are made use of in classification, and in the definitions of genera and larger groups.
Sub-Order 2.—Ascidiae Compositae.
Fixed Ascidians which reproduce by gemmation, so as to form colonies in which the ascidiozooids are buried in a common investing Compound Ascidians. mass and have no separate tests. This is probably a somewhat artificial assemblage formed of two or three groups of Ascidians which produce colonies in which the ascidiozooids are so intimately united that they possess a common test or investing mass. This is the only character which distinguishes them from the Clavelinidae, but the property of reproducing by gemmation separates them from the rest of the Ascidiae Simplices. The Ascidiae Compositae may be divided into seven families, which fall into two well-marked groups: (1) the Chalarosomata, including the first five families, with extended body, divided into two or three regions, and more nearly related to the Clavelinidae; and (2) the Pectosomata, including the Botryllidae and Polystyelidae, with a compact body, not divided into regions, and evidently related to the Cynthiidae amongst simple Ascidians.
Family I., Distomidae.—Ascidiozooids divided into two regions, thorax and abdomen; testes numerous; vas deferens not spirally coiled. The chief genera are: Distoma (Gaertner); Distaplia (Della Valle); Colella (Herdman), forming a pedunculated colony (see fig. 28, A) in which the ascidiozooids develop incubator pouches, connected with the peribranchial cavity, in which the embryos undergo their development (17); and Chondrostachys (Macdonald).
Family II., Coelocormidae.—Colony not fixed, having a large axial cavity with a terminal aperture. Branchial apertures five-lobed. This includes one species, Coelocormas huxleyi (Herdman), which is, in some respects, a transition form between the ordinary compound Ascidians (e.g. Distomidae) and the Ascidiae Luciae (Pyrosoma).
| |||||||||
(After Herdman, “Challenger” Report.) | |||||||||
| Fig. 28.—Colonies of Ascidiae Compositae. (Natural size.) | |||||||||
|
Family III., Didemnidae.—Colony usually thin and in crusting test containing stellate calcareous spicules. Testis single, large; vas deferens spirally coiled. The chief genera are—Didemnum (Savigny), in which the colony is thick and fleshy and there are only three rows of stigmata on each side of the branchial sac; and Leptoclinum (Milne-Edwards), in which the colony is thin and incrusting (fig. 28, B) and there are four rows of stigmata on each side of the branchial sac.
Family IV., Diplosomidae.—Test reduced in amount, rarely containing spicules. Vas deferens not spirally coiled. In Diplosoma (Macdonald), the most important genus, the larva is gemmiparous.
Family V., Polyclinidae.—Ascidiozooids divided into three regions—thorax, abdomen and post-abdomen. Testes numerous; vas deferens not spirally coiled. The chief genera are: Pharyngodictyon (Herdman), with stigmata absent or modified, containing one species, Ph. mirabile (fig, 28, C), the only compound Ascidian known from a depth of 1000 fathoms; Polyclinum (Savigny), with a smooth-walled stomach; Aplidium (Savigny), with the stomach wall longitudinally folded (fig. 27); and Amaroucium (Milne-Edwards), in which the ascidiozooid has a long post-abdomen and a large atrial languet.
|
(After Pizon.) |
|
Fig. 29.—Young Colony of Botryllus, showing Buds and Ampullae. |
Family VI., Botryllidae.—Ascidiozooids having the intestine and reproductive organs alongside the branchial sac. Dorsal lamina present; internal longitudinal bars present in branchial sac. The chief genera are: Botryllus (Gaertn. and Pall.), with simple stellate systems (fig. 28, D), and Botrylloides (Milne-Edwards), with elongated or ramified systems. It is well known that in the family Botryllidae, amongst compound Ascidians, the ectodermal vessels containing blood, which ramify through the common test and serve to connect the vascular systems of the various members of the colony, have numerous large ovate dilatations, the ampullae, upon their terminal twigs (fig. 29). Various functions have been assigned to these ampullae in the past, and Bancroft has shown that in addition to acting as storage reservoirs for blood, organs for the secretion of test matrix, and accessory organs of respiration, they are also organs for blood propulsion. The ampullae execute co-ordinated pulsations, the co-ordination being due to variations in the blood-pressure. It was actually found that the ampullae could keep up the circulation for some time in a portion of a colony independently of the hearts of the ascidiozooids. All the hearts in a colony of Botryllus contract simultaneously and in the same direction. The reversal of the circulation may be regarded as due to the engorgement of the ampullae in the superficial parts of the colony. These when distended overcome the resistance of the heart's action, and cause it to stop and then reverse.
Family VII., Polystyelidae.—Ascidiozooids not grouped in systems. Branchial and atrial apertures four-lobed. Branchial sac may be folded; internal longitudinal bars present. The chief genera are: Thylacium (Carus), with ascidiozooids projecting above general surface of colony; Goodsiria (Cunningham), with ascidiozooids completely imbedded in investing mass; and Chorizocormus (Herdman), with ascidiozooids united in little groups which are connected by stolons. Several of the species show transitions between the other Polystyelidae and the Styelinae amongst simple Ascidians.
Gemmation and Growth of Colonies.—A number of new observations have been made in recent years upon the budding of compound Ascidians, some of which are very puzzling and contradictory in their results. Metschnikoff, Kowalevsky, Giard, Hjort, Pizon, Seeliger, Ritter, van Beneden and Julin have all in turn added to our knowledge of the details of development and life-history, of the various processes of gemmation and of the formation of colonies. It is impossible as yet to reconcile all the conflicting accounts, but the following points at least seem pretty clear.
Gemmation may be very different in its details in closely related compound Ascidians. There are, however, two main types of budding, to one or other of which most of the described methods may be referred. There is first the “stolonial” or “epicardiac” type, seen in the Chalarosomata, typically in Distomidae and Polyclinidae, and comparable with the gem mat ion in Clavelinidae, Pyrosomidae and Thaliacea outside this group. Secondly, there is the “parietal” or “peribranchial” type, seen in the Pectosomata, typically in the Botryllidae. The remarkable process of gemmation seen in the families Didemnidae and Diplosomidae may probably be regarded as a modification of the stolonial type. The double embryo in the Diplosomidae is probably to be intenzpreted as precocious budding (rather than as embryonic fission), due to acceleration in development (tachygenesis). The type of budding, and even details such as the length of the stolon, have much to do with differences in the nature and appearance of the colonies produced. The stolon, which has a wall continuous with the body-wall of the parent, contains an endodermal element in the form of the so-called “epicardium,” and also a prolongation of the ovary, or at least a string of migrating germ-cells, so that the reproductive elements are also handed on. Still, it is clear from recent researches that the development of the bud (blastozooid) and that of the embryo (oozooid) do not proceed along parallel lines. It is impossible to harmonize the facts of gemmation with the germ-layer theory, and attempts to explain budding in Ascidians as a process of regeneration, by which the organs of the parent or their germ-layers give rise to the corresponding organs in the bud, have signally failed.
| ||||||||||
(After Pizon.) | ||||||||||
|
Fig. 30.-Young buds of Botryllus sectioned to show the separation of the branchial (vb) from the peribranchial (cp) cavities. | ||||||||||
|
Figs. 29 and 30 show the buds in the Botryllidae, after Pizon, who has followed day by day the changes of growth in young colonies of Botryllidae, tracing the rise of successive generations of buds and the degeneration of their parents. The buds are parietal, arising from the walls of the peribranchial cavities (fig. 29), and at an early period they acquire the structure shown in fig. 30, where there are two vesicles undergoing further subdivision and differentiation, but investigators still differ as to whether the inner, which gives rise to the branchial sac and alimentary canal, is not produced along with the outer from the ectoderm of the parent.
A remarkable case of polymorphism has been found by M. Caullery in the buds of the compound Ascidian Colella. Some of the buds Reproduction by Gemmation and the Formation of Colonies. have an abundant store of reserve materials in their outer layer of cells, while others are without this supply. The former are placed deeply in the stalk, develop slowly, and probably serve to regenerate the colony when the head portion has been removed or has died down. In these cases where the ectoderm has taken on the function of storing the reserve material, it is found that all the organs of the bud are formed from the cells of the endodermic vesicle. The first ascidiozooid of the colony produced by the tailed larva does not form sexual reproductive organs, but reproduces by gemmation so as to make a colony. Thus there is alternation of generations in the life-history. In the most completely formed colonies (e.g. Botryllus) the ascidiozooids are arranged in groups (systems or coenobii), and in each system are placed with their atrial apertures towards one another, and all communicating with a common cloacal cavity which opens to the exterior in the centre of the system (fig. 28, D).
Sub-Order 3.-Ascidiae Luciae.
|
|
Fig. 31.—Pyrosoma elegans. |
| A, Side view of entire colony. |
| B, End view of open extremity. |
Free-swimming pelagic colonies having the form of a hollow cylinder closed at one end. The ascidiozooids forming the colony are Ascidiae Luciae. embedded in the common test in such a manner that the branchial apertures open on the outer surface and the atrial apertures on the inner surface next to the central cavity of the colony. The ascidiozooids are produced by gemmation from a rudimentary larva (the cyathozooid) developed sexually.
This sub-order includes a single family, the Pyrosomidae, containing Structure of Pyrosoma. one well-marked genus, Pyrosoma (Péron), with half a dozen species. They are found swimming near the surface of the sea, chiefly in tropical latitudes, and are brilliantly phosphorescent. A fully developed Pyrosoma colony may be from an inch or two to upwards of twelve feet in length. The shape of the colony is seen in fig. 31. It tapers slightly towards the closed end, which is rounded. The opening at the opposite end is reduced in size by the presence of a membranous prolongation of the common test (fig. 31, B). The branchial apertures of the ascidiozooids are placed upon short papillae projecting from the general surface, and most of the ascidiozooids have long conical processes of the test projecting outwards beyond their branchial apertures (figs. 31, 32 and 33). There is only a single layer of ascidiozooids in the Pyrosoma colony, as all the fully developed ascidiozooids are placed with their antero-posterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity of the colony (fig. 32). Their dorsal surfaces are turned towards the open end of the colony. The more important points in the structure of the ascidiozooid of Pyrosoma are shown in fig. 33. A circle of tentacles, of which one, placed ventrally (fig. 33, tn), is larger than the rest, is found just inside the branchial aperture. From this point a wide cavity, with a few circularly placed muscle bands running round its walls, leads back to the large branchial sac, which occupies the greater part of the body. The stigmata are elongated transversely and crossed by internal longitudinal bars. The dorsal lamina is represented by a series of eight languets (l). The nerve ganglion (on which is placed a small pigmented sense organ), the subneural gland, the dorsal tubercle, the peripharyngeal bands, and the endostyle are placed in the usual positions. On each side of the anterior end of the branchial sac, close to the peripharyngeal bands, is a mass of rounded gland cells which are the source of the phosphorescence. The alimentary canal is placed posteriorly to the bronchial sac, and the anus opens into a large peribranchial (or atrial) cavity, of which only the median posterior part is shown (p.br.) in fig. 33. The reproductive organs are developed in a diverticulum of the peribranchial cavity, and consist of a lobed testis and a single ovum at a time. The development takes place in a part of the peribranchial cavity (Fig. 32, em). The segmentation Development of Pyrosoma. is meroblastic, and an elongated embryo is formed on the surface of a mass of yolk. The embryo, after the formation of an alimentary cavity, a tubular nervous system, and a pair of laterally placed atrial tubes, divides into an anterior and a posterior part. The anterior part then segments into four pieces, which afterwards develop into the first ascidiozooids of the colony, while the posterior part remains in a rudimentary condition, as the “cyathozooid”; it eventually atrophies. As the four ascidiozooids increase in size, they grow round the cyathozooid and soon encircle it (fig. 32, asc and cy). The cyathozooid absorbs the nourishing yolk upon which it lies, and distributes it to the ascidiozooids by means of a heart and system of vessels which have been meanwhile formed. When the cyathozooid atrophies and is absorbed, its original atrial aperture remains and deepens to become the central cavity of the young colony, which now consists of four ascidiozooids placed in a ring, around where the cyathozooid was, and enveloped in a common test. The colony gradually increases by the formation of buds from these four original ascidiozooids.
| |||||||||||||||||||
| (Partly after Savigny.) | |||||||||||||||||||
|
Fig. 32.—Part of a Longitudinal Section through wall of Pyrosoma, showing arrangement of ascidiozooids, magnified. | |||||||||||||||||||
|
| |||||||||||||||||
| (Partly after Keferstein.) | |||||||||||||||||
|
Fig. 33.-Mature Ascidiozooid of Pyrosoma, from left side. | |||||||||||||||||
|
Phylogeny
The accompanying diagram (fig. 34) shows graphically the probable origin and course of evolution of the various groups of Tunicata, Phylogeny. and therefore exhibits their relations to one another much more correctly than any system of linear classification can do. The ancestral Proto-Tunicata are here regarded[8] as an offshoot from the Proto-Chordata—the common ancestors of the Tunicata (Urochorda), Amphioxus (Cephalochorda) and the Vertebrata. The ancestral Tunicata were probably free-swimming forms, not very unlike the existing Apendiculariidae, and are represented in the life-history of nearly all sections of the Tunicata by the tailed larval stage. The Larvacea are the first offshoot from the ancestral forms which gave rise to the two lines of descendants, the Proto-Thaliacea and the Proto-Ascidiacea. The Proto-Thaliacea then split into the ancestors of the existing Cyclomyaria and Hemimyaria. The Proto-Ascidiacea gave up their pelagic mode of life and became fixed. This ancestral process is repeated at the present day when the free-swimming larva of the simple and compound Ascidians becomes attached. The Proto-Ascidiacea, after the change, are probably most nearly represented by the existing genus Clavelina. They have given rise directly or indirectly to the various groups of simple and compound Ascidians and the Pyrosomidae. These groups form two lines, which appear to have diverged close to the position of the family Clavelinidae. The one line leads to the more typical compound Ascidians, and includes the Polyclinidae, Distomidae, Didemnidae, Diplosomidae, Coelocormidae, and finally the Ascidiae Luciae or Salpiformes. The second line gave rise to the simple Ascidians, and to the Botryllidae and Polystyelidae, which are, therefore, not closely allied to the other compound Ascidians. The later Proto-Ascidiacea were probably colonial forms, and gemmation was retained by the Clavelinidae and by the typical compound Ascidians (Distomidae, &c.) derived from them. The power of forming colonies by budding was lost, however, by the primitive simple Ascidians, and must, therefore, have been regained independently by the ancestral forms of the Botryllidae and the Polystyelidae. If this is a correct interpretation of the of evolution of the Tunicata, we arrive at the following important conclusions. (1) The Tunicata, as a whole, form a degenerate branch of the Proto-Chordata; (2) the Ascidiae Luciae (Pyrosoma) are much more closely related to the typical compound Ascidians than to the other pelagic Tunicata, viz. the Larvacea and the Thaliacea; and (3) the Ascidiae Compositae, form a polyphyletic group the sections of which have arisen at several distinct points from the ancestral simple Ascidians.
Fig. 34.
Bibliography.—(1) Cuvier, “Mém. s. les Ascidies,” &c., in Mém. d. Mus. ii. 10 (Paris, 1815); (2) Savigny, Mémoires sur les animaux sans vertèbres, pt. ii. fasc. i. (Paris, 1816); (3) Lamarck, Hist. nat. d. anim. sans vertèbres (1st ed., Paris, 1815-1823); (4) O. F. Müller, Zool. danica. (1806), vol. iv.; (5) Milne-Edwards, “Observ. s. les Ascidies Composées,” &c., in Mém. Acad. Sci. vol. xviii. (Paris, 1842); (6) Schmidt, Zur vergl. Physiol. d. wirbellos. Thiere (Brunswick, 1845); (7) Löwig and Kölliker, “De la Compos., &c., d. Envel. d. Tun.,” in Ann. Sc. Nat., 1846 (Zool.), 3rd series, vol. v.; (8) Huxley, Phil. Trans. (1851); (9) Kowalevsky, “Entwickel. d. einf. Ascid.,” in Mem. St Petersb. Acad. Sc. (1866), 7th series, vol. x.; (10) J. P. van Beneden, “Rech. s. l'Embryolog., &c., d. Asc. Simp.,” in Mém. acad. roy. belg. (1847), vol. xx.; (11) Krohn, in Wiegmann and Müller's Archiv (1852); (12) Kupffer, Arch. f. mikr. Anal. (1869, 1872); (13) Giard, “Étude d. trav. embryolog. d. Tun., &c.,” in Arch. zool. expér. (1872), vol. i.; (14) Fol, “Études sur les appendiculaires du détroit de Messine,” in Mém. soc. phys. hist. nat. Genève, vol. xxi.; (15) Giard, “Recherches s. l. Asc. Comp.,” in Arch. zool. expér. (1872), vol. i.; (16) Von Drasche, Die Synascidien der Bucht von Rovigno (Vienna, 1883); (17) Herdman, “Report upon the Tunicata of the ‘Challenger’ Expedition,” pt. i. in Zool. “Chall.” Exp. (1882), vol. vi.; pt. ii. in Zool. “Chall.” Exp. (1886), vol. xiv.; pt. iii. in Zool. “Chall.” Exp. (1889), vol. xxvii.; (18) Alder and Hancock, in Ann. Mag. Nat. Hist. (1863, 1870); (19) Heller, “Untersuch. u. d. Tunic. d. Adriat. Meeres,” in Denkschr. d. k. Akad. Wiss. (1875-1877); (20) Lacaze-Duthiers, “Asc. simp. d. côtes d. l. Manche,” in Arch. zool. expér. (1874, 1877); (21) Traustedt, in Vidensk. medd. naturh. For. (Copenhagen, 1881-1884); (22) Herdman, “Notes on British Tunicata, &c.,” in Journ. Linn. Soc. Zool. (1880), vol. xv.; (23) Ussoff, in Proc. imp. soc. nat. hist. (Moscow, 1876), vol. xviii.; (24) julin, “Rech. s. l'org. d. asc. simp.,” in Arch. d. biol. (1881), vol. ii.; (25) Brooks, “Development of Salpa,” in Bull. Mus. Comp. Zool. iii. 291 (Harvard); (26) Salensky, Ztschr. f. wiss. Zool. (1877); (27) Barrois, Journ. d. l'anat. et phys. (1885), vol. xxi.; (28) Uljanin, Fauna, &c., d. Golfes von Neapel (1884), vol. x.; (29) Moseley, “On Deep-sea Ascid,” in Trans. Linn. Soc. (1876), 2nd series, vol. i.; (30) E. van Beneden and Julin, “Morph. d. Tuniciers,” in Arch. d. Biol. (1886), vol. vi.; (31) Dohrn, “Studien zur Urgesch. der Wirbelth.” in Mitth. zool. Stat. Neapel; (32) Herdman, “Revised Classification,” Journ. Linn. Soc. (1891), vol. xxiii.; (33) Herdman, Descriptive Catalogue of Australian Tunicata (1899); (34) Brooks, The Genus Salpa (1893); (35) Seeliger, Bronn's Thier-Reich Tunicata. (W. A. He.)
- ↑ Only the more important works can be mentioned here. For a more detailed account of the history of the group and a full bibliography see (17) and (35) in the list of works at the end of this article.
- ↑ According to E. van Beneden and Julin (30) only the outer wall of the atrium is lined with epiblast, the inner wall being derived from the hypoblast of the primitive branchial sac.
- ↑ On account of the periodic reversal of the circulation none of the vessels can be called arteries or veins.
- ↑ For structure of other forms, see below.
- ↑ For reproduction by gemmation see under “Classification” below.
- ↑ Some of the first test cells are also probably derived from the epithelium of the egg follicle.
- ↑ For further details see Uljanin (28) and Neumann, Doliolum, in Deutsch. Tief-See Exped. (Jena, 1905).
- ↑ By Dohrn and others their point of origin is placed considerably farther up on the stem of the Chordata, thus causing the Tunicata to be regarded as very degenerate Vertebrata (see 31).