(From The Cambridge Natural
See also:History, vol. vii., " Fishes, &c." By permission of
See also:Macmillan & Co., Ltd.) Kowalevsky and others) . A,
See also:Free-swimming tailed larva . B, The metamorphosis—larva attached . C, Tail and
See also:system of larva degenerating . D, Further degeneration and
See also:metamorphosis of larva into E, the
See also:young fixed Ascidian . at, Atrial invagination. m, Mouth . ch, Notochord.
See also:mes, Mesenteron . hy, Hypoblast cells. nc, Neural canal. i,
See also:Intestine . in, Neural vesicle with sense-
See also:ORDER I.—LARVACEA Free-swimming pelagic forms provided with a large locomotory appendage (the tail), in which there is a skeletal
See also:axis (the urochord) . A relatively large test (the "
See also:house ") is formed with characters
See also:great rapidity as a secretion from the ectoderm ; it is of Larvacea. merely a temporary structure, which is
See also:cast off and replaced by another . The branchial
See also:sac is simply an enlarge(' pharynx with two ventral cili- ated openings (stigmata) leadin to the exterior . There is no
See also:separate peribranchial cavity .
The nervous system consists of a large dorsally placed ganglion and along nerve
See also:cord, which stretches backwards over the alimentary canal to reach the tail, along which it runs on the
See also:left side of the urochord . The anus opens ventrally on the
See also:surface of the
See also:body in front of the stigmata . No
See also:reproduction by gemmation or metamorphosis is known in the
See also:life-history . This is one of the most interesting groups (fig . 16) of the
See also:Tunicata, as it Structure of shows more corn- Appendlcupletely than any of lane . i the
See also:rest the
See also:char- acters of the
See also:original ancestral forms . It has undergone little or no degeneration, and consequently corresponds more nearly to the tailed-larval
See also:condition than to the adult forms of the other groups . The order includes a single
See also:family, the Appendiculariidae, all the members of which are minute and free-swimming . They occur on the surface of the
See also:sea in most parts of the
See also:world . They possess the power to
See also: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
See also:volume of
See also:water is strained so as to entrap microscopic
See also:food particles; it
See also:helps in locomotion by its hydrostatic effect, and it is also a
See also:protection to the animal, which may
See also:escape from enemies by throwing off the house, which is many times its own
See also: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
See also: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
See also: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
See also:short . There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly .
See also:wall of the branchial sac has only two ciliated apertures (fig . 19) . They are homologous with the
See also: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
See also:line, and lead into short
See also: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
See also:band . Cerebral ganglion . Caudal ganglion . Enlargement of nerve cord in tail . Sense-
See also:organ (tactile) on
See also: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
See also:remainder of the structural details can be made out from
See also:figs . 18 and 19 . At, Atrial passage. n, Nerve . b.s,
See also:Blood sinus. n.ch, 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
See also: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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