PHORONIDEA, a zoological order, containing a single genus Phoronis, which is known to be of practically world-wide distribution, while there are many records of its larva, Actinotrocha, from localities where the adult has not been found. Phoronis is often gregarious, the tubes which it secretes being sometimes intertwined in an inextricable mass. These associations of individuals can hardly be the result of the metamorphosis of a corresponding number of larvae, but are probably due to a spontaneous fragmentation of the adult animals, each such fragment developing into a complete Phoronis (De Selys-Longchamps). The animal is from a quarter of an inch to six inches (P. australis) in length. The free end of the long vermiform body ends in a horseshoe-shaped “lophophore,” or tentacle bearing region (fig. 1, a), which strikingly resembles that of the Phylactolaematous Polyzoa (see Polyzoa).
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Fig. 1.—The Tentacular End of Phoronis, with most of the tentacles removed. | ||||||||||||||||||||
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Fig. 2.—Dorsal View of Phoronis australis, showing the spirally coiled ends of the lophophore. | ||||||||||||||||||||
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In some species (figs. 2, 3) the two ends of the lophophore are rolled into spirals. An oral view of this region (fig. 2) shows: the mouth (m), continuous on either side with the groove between the two series of tentacles; the anus (a), in the middle line, at no great distance from the mouth; a transversely elongated epistome (ep), between the mouth and the anus; and, in the concavity of the lophophore, the apertures of the nephridia (n.o.) which, according to De Selys-Longchamps, open into the two large sensory or glandular “lophophoral organs” the orifices of which are seen at gl. The mouth leads into the oesophagus, which extends straight down the body nearly to the aboral end or “ampulla,” where it dilates into a stomach, from which the ascending limb of the U-shaped alimentary canal passes directly to the anus. The coelomic body-cavity is divided by a transverse septum (fig. 3, 5) which lies near the bases of the tentacles. The praeseptal or lophophoral coelom is continued into each of the tentacles and into the epistome. The postseptal coelom is partially divided by a ventral mesentery which is attached along the entire length of the convex side of the loop of the alimentary canal (a, a″) and by two lateral mesenteries (a′) which further connect the oesophagus with the body-wall. Each nephridium is provided with either one or two funnels which open into the postseptal division of the coelom (ne f). The nervous system lies in the epidermis, externally to the basement-membrane. A general nerve-plexus probably exists over considerable parts of the skin, and there are special nervous concentrations in the region of the epistome and along a double crescent (N) which follows the parietal attachment of the coelomic septum. The part which lies at the base of the epistome is morphologically dorsal in position. It is said by Schultz (11) to develop, in specimens which are regenerating the lophophoral end, from an invaglnation of the ectoderm: and in this condition is compared by him with the hollow central nervous system of some Enteropneusta and of Vertebrates. This comparison is not admitted by De Selys-Longchamps. The vascular system contains numerous red blood-corpuscles. The principal blood-channels are two longitudinal vessels which run down the entire length of the body, and are known as the “afferent” vessel (af) and the “efferent” vessel (ef) respectively, from their relation to the tentacles. According to researches in 1907 by De Selys-Longchamps, the blood is driven by the afferent vessel (af) to a crescentic lophophoral vessel (d.v.) which supplies the tentacles. Each of these contains a single blindly ending vessel which bifurcates at its base (see fig. 3). One of these branches communicates with the afferent lophophoral vessel, while the other one opens into the crescentic efferent lophophoral vessel (r.v.). From this the blood passes into two lateral vessels which pierce the coelomic septum (s), the right vessel proceeding on the anterior side of the oesophagus, as shown in fig. 3, to effect a union with the left one, and thus to constitute the main efferent vessel, which gives off numerous caecal branches as it passes down the body. Hence the blood returns once more to the afferent vessel through a splanchnic sinus which surrounds the stomach. The circulation is maintained by the rhythmical contraction of the afferent vessel and by less regular contractions of some of the other vessels. The reproductive organs lie on the left side, near the aboral end, both ovary and testis being present in the same individual in some of the species. They are said to be developed from the coelomic epithelium which covers the efferent vessel or its caeca. The reproductive cells pass to the exterior by means of the nephridia. Reproduction by budding does not occur, although spontaneous fragmentation of the body, followed by complete regeneration of each of the pieces, is known to take place. Regeneration of the tentacular end of the animal is of frequent occurrence.
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Fig. 3.—Diagram of oral end of Phoronis australis, seen from the left side. | ||||||||||||||||||||||||||||||||||||||||||||
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Development and Affinities.—The eggs of Phoronis are small and usually undergo their early development attached to the tentacles of the adult. The attachment is probably effected (Masterman) by the secretion of the lophophoral organs (fig. 2, gl). After the formation of an invaginate gastrula the larval form is rapidly acquired. On quitting the shelter of the parent tentacles the embryo becomes a pelagic larva, known as Actinotrocha (fig. 4) characterized by the possession of a line of tentacles running obliquely round the body. Locomotion is effected principally by means of a posterior ring of cilia surrounding the anus. The mouth (o) is in front of the tentacles, on the ventral side, and is overhung by a mobile praeoral hood, in which is the principal part of the nervous system. An oblique septum which follows the bases of the tentacles and corresponds with that of the adult animal divides the body-cavity into two portions. The postseptal division is a coelomic space, partially subdivided by a ventral mesentery. The praeseptal cavity is a vascular space, since it is in free communication with the dorsal vessel of the larva, and it persists in part as the two lophophoral vascular crescents of the adult. It contains two tufts of peculiar excretory cells, described by Goodrich (5) as “solenocytes,” which surround the blind ends of a pair of nephridia. These pass backwards through the septum and open to the exterior ventrally. After the Actinotrocha has led a pelagic life for some time it develops a large ventral invagination of its body-wall (fig. 4, 2, iv.). At the metamorphosis, this sac is everted and the alimentary canal is drawn into it in the form of a loop (fig. 4, 3, 4). Most of the praetentacular region and the larval tentacles separate off, being then taken into the alimentary canal, where they are digested. The relations of the surfaces after the metamorphosis are clearly very different from those which obtained in the larva. The dorsal surface of the adult is the one between the mouth and the anus, while the median ventral line is the one which corresponds with the convexity of the alimentary canal. This view of the surfaces is, however, disputed by De Selys-Longchamps, who regards the aboral extremity of the adult as the posterior end.
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Fig. 4.—Diagrams illustrating the Metamorphosis of Actinotrocha. | ||||||||||
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The development of Phoronis was supposed by Caldwell (2) to furnish the explanation of the relations of the surfaces in Brachiopoda, Polyzoa and perhaps the Sipunculoid Gephyrea, in which the ontogenetic evidence is less clear. Caldwell's views were accepted by Lankester (8) in the 9th edition of this work, the Phylum Podaxonia being there instituted to include the groups just mentioned, together with the Pterobranchia. The peduncle of the Brachiopoda was supposed to correspond with the everted ventral sac of Actinotrocha, but the question is complicated by the want of any complete investigation of the development of the Brachiopoda, and by the absence of the anus in the majority of the genera. There is, however, a considerable amount of resemblance between the lophophore of Phoronis australis, with its spirally twisted ends, and that of a typical Brachiopod; nor do the structural details of the adult Brachiopods forbid the view that they may be related to Phoronis. The comparative study of the development does not support the hypothesis that the Polyzoa (q.v.) are comparable with Phoronis. In Pedicellina, the only Polyzoon in which the alimentary canal of the larva is known to become that of the first adult individual, the line between the mouth and anus is ventral in the larva; and since there is no reversal of the curvature of the digestive loop during the metamorphosis it must be regarded as ventral in the adult. There are, indeed remarkable similarities between the external characters of the Phylactolaematous Polyzoa and the Phoronidea, and notably between their lophophores. The supposed occurrence of a pair of nephridia in certain Phylactolaemata, in a position corresponding with that of the nephridia of Phoronis, must also be mentioned, although it has been maintained that the “nephridia” of Phylactolaemata are merely ciliated portions of the body-cavity and not indeed nephridia at all. But a serious objection to the comparison is that the development of Phylactolaemata can be explained by supposing it to be a modification of what occurs in other Polyzoa, while if appears to have no relation whatever to that of Phoronis.
Most observers consider that Actinotrocha is a highly modified Trochosphere, and this would give it some claim to be regarded as distantly related to the Entoproct Polyzoa and to other groups which have a Trochosphere larva.
Phoronis has long been regarded as a possible ally of Rhabdopleura (see Pterobranchia); and Masterman (10) has attempted to demonstrate the existence in Actinotrocha of most of the structures which occur in the Pterobranchia. According to his view the praeoral hood of Actinotrocha (cf. fig. 4) corresponds with the “proboscis” of Pterobranchia; the succeeding region, as far as the bases of the tentacles, with the collar; and the post-tentacular region with the metasome. Masterman’s more detailed comparisons have for the most part been rejected by other morphologists. One of the most formidable difficulties in the way of the attempt to reduce Actinotrocha to the Pterobranchiate type of structure is the condition of the coelom in the former. There is indeed a perfectly definite transverse septum which divides the body-cavity in the region of the tentacle-bases. Even if it be admitted that the postseptal space may be the metasomatic cavity, the praeseptal space can hardly be regarded as coelomic in nature, since it is in continuity with the vascular system; while Masterman’s conclusion that the cavity of the praeoral hood (the supposed proboscis-cavity) is separated from that of the supposed collar has received no confirmation. In spite of these difficulties it must be conceded that the dorsal flexure of the alimentary canal of the Pterobranchia is very Phoronis-like. It has, moreover, been shown (see especially Goodrich, 5) that shortly before its metamorphosis, Actinotrocha develops a coelomic space which lies immediately in front of the oblique septum, and gives rise later to the cavity of the lophophore and tentacles. Regarding this as a collar-cavity, it becomes possible to agree with Masterman that the region shown in fig. 4, 1. between the tentacles and the praeoral hood, is really a collar the coelom of which develops relatively late. It will be noticed that the lophophore of Phoronis is, on this assumption, a derivative of the collar just as it is in the Pterobranchia. The epistome of the adult Phoronis cannot well be the proboscis since its cavity is continuous with the lophophoral coelom, and because the praeoral hood of Actinotrocha is entirely lost at the metamorphosis. It is possible that this consideration will account for the want of an anterior body-cavity in Phoronis. Since the proboscis is a purely larval organ in this genus it may be supposed that the coelomic space which properly belongs to it fails to develop, but that the praeoral hood itself is none the less the morphological representative of the proboscis. In spite of the criticisms which have been made on the conclusion that Phoronis is allied to the Pterobranchia, it is thus possible that the view is a sound one, and that the Phoronidea should take their place, with the Enteropneusta and the Pterobranchia, as an order of the Hemichordata.
Bibliography.—(1) Benham, Quart. Journ. Mic. Soc. xxx. 125 (1890), (2) Caldwell, Proc. Roy. Soc. xxxiv. 371 (1883); (3) Cori, Zeitschr wiss. Zool. li. 480 (1891); (4) Fowler, art. “Hemichorda,” Ency. Brit. xxix. 249 (1902); (5) Goodrich, Quart. Journ. Mic. Soc. xlvii. 103 (1904); (6) Harmer, Siboga Rep. xxvi. 114, bis (Pterobranchia), (1905); (7) Ikeda, J. Coll. Sci. Japan, xiii. 507 (1901); (8) Lankester, art. “Polyzoa,” Ency. Brit. xix. 430, 433 (1885); (9) De Selys-Longchamps, Arch. Biol. xviii. 495 (1902); Wiss. Meeresunt. (N. F.) vi. Abt Helgoland (1903), Heft i.; Mém classe sci. acad. belgique, vol i. (1904); Fauna u. Flora G. v. Neapet, 30 Monogr. (1907); (10) Masterman, Quart Journ. Mic. Soc. xl. 281 (1898); xliii. 375 (1900); (11) Schultz, Zeitschr. wiss. Zool. lxxv. 391, 473 (1903); (12) Shearer, Mitth. zool. Stat. Neapel, xvii. 487 (1906); 13) Shipley, Cambr. Nat. Hist. ii. 450 (1896). (S. F. H.)