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Online Encyclopedia
Originally appearing in Volume V27, Page 386 of the 1911 Encyclopedia Britannica.
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CLASSIFICATION AND CHARACTERS OF GROUPS (From The Cambridge Natural History, vol. vii., " Fishes, &c." By permission of Macmillan & Co., Ltd.) 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. at, Atrial invagination. m, Mouth. ch, Notochord. mes, Mesenteron. hy, Hypoblast cells. nc, Neural canal. i, Intestine. in, Neural vesicle with sense-organs.ORDER I.—LARVACEA Free-swimming pelagic forms provided with a large locomotory appendage (the tail), in which there is a skeletal axis (the urochord). A relatively large test (the " house ") is formed with characters great rapidity as a secretion from the ectoderm ; it is of Larvacea. merely a temporary structure, which is cast off and replaced by another. The branchial sac is simply an enlarge(' pharynx with two ventral cili- ated openings (stigmata) leadin 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 Tunicata, as it Structure of shows more corn- Appendlcupletely than any of lane. i the rest the char- acters 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. II), 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. I.ohmann 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 branchial 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 (From The Cambridge Natural History, vol. vii.," Fishes," &c. By permission of Macmillan & Co., Ltd.) (After Fol.) x, Lateral reticulated parts of " House." 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 Ovary. Peripharyngeal band. Cerebral ganglion. Caudal ganglion. Enlargement of nerve cord in tail. Sense-organ (tactile) on lower lip. Ciliated aperture in pharynx. Stomach. Testis. Urochord. Its cut end. 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. At, Atrial passage. n, Nerve. b.s, Blood sinus., Notochord. br.s, Branchial sac (pharynx). R, Rectum. ec, Ectoderm. sg, Stigma. en, Endoderm. t, Test. 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 bod ; and Kowalevskia (Fol), a remarkable form described by Fol body; 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.
CLASSIFICATION (Lat. classis, a class, probably fro...

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