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
