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Encyclopedia Britannica - Main :: ORC-PAI |
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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 a notochord. The test is permanent and may be either Thallacea. well developed or very slight. The musculature of the mantle is in the Corm 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 i.Cyclomyaria. Free-swimming pelagic forms which exhibit alternation of generations in their life-history but never form permanent colonies. The body
tures characters at the opposite ends. The test is more or less well of developed. The mantle has its musculature in the form ofCvc1. of circular bands surrounding the body
sac is fairly large, occupying the anterior half or more of the body. Stigmata are usually present in its posterior part only. The pertbranchial 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). 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 branchial and atrial apertures (fig. 20) are structure oI lobed and the lobes are provided with sense organs. Dololum. 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 branchial and atrial sphincters. The wide branchial and atrial apertures lead into large branchial and peribranchial cavities, separated by the posterior wall
axis
g1 fn rna m Fig. 18.Semi-diagrammatic view of Appendicularia from the right. a, Anus. at, One of the atrial apertures. app, Tail. br, Branchial aperture. brs, Branchial sac. dl, Dorsal tubercle. end. Endostyle. _ so, Is, Heart. i, Intestine. sg, m, Muscle band of tail. n, Nerve cord in body. st, n', Nerve cord in the tail. tes, oe, Oesophagus. u, ot, Otocyst. u', ov, pp, ng, ng', ng,, ao` .erid si s . br''s tes k at m'm8, Muscle bands. pbr, Peribranchial cavity. ng, Nerve ganglion. atl, Atrial lobes. sg, Stigmata. so, Sense organs. sgl, Subneural gland. brl, Branchial lobes. .,11,1 1. I 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 characterized by the possession of nine muscle bands, D el Dollolum. auditory sac on the left side of the body, a ventrally- of 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
young
Anchinia, of which only one species is known, A. rubra, from the Mediterranean, has the sexual forms permanently attached to Anc6lnoa. 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 The body is more or less fusiform, with the long axis
of Hemp antero-posterior, and the branchial and atrial apertures myarla. 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, oras is more usualbe 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.This sub-order contains one family, the Salpidae, includingthesingle Sal ldae. genus Salpa (Forsk$l), which, however, may be divided into two well-marked groups of species(I) those, such as S. pinnata, in which the alimentary canal is stretched out along the ' For further details see Uljanin (28) and Neumann, Doliolum, in Deutsch. Tief-See Exped. (Jena, 1905).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 A B A, Aggregated form: em, Embryo; gem, Gemmiparous stolon; m, Mantle; visc, Visceral mass (nucleus). B, Solitary form: i-9, Muscle bands. Lettering as before. altogether are known; they are all pelagic forms and are found in nearly all seas. Each species occurs in two formsthe 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 diverticulumfrom the mantle so as to form FIG. 22.Posterior part of " chains " (fig. 22). Each member solitary form of Salpa demo-of the chain is a Salpa of the sexual cratica-mucronata, showing a or aggregated form, and when mature chain of embryos nearly ready mayeither still attached to its to be set free. neighbours or separated from them gem, Young aggregated Salpae (fig. 21, A)produce one or several forming the chain. embryos, which develop into the st, Stolon. solitary Salpa. Thus the two forms m, Muscle band of the mantle. alternate regularly. The more important points in the structure of a typical Salpa are shown in fig. 23. The branchial and atrial apertures are at opposite ends of the body, and each leads into a large cavity, structure of the branchial and peribranchial sacs, which are in free saopa. 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 gel in ! emb Q. f; a 13 5 6; 7 8 y rn ii b tes emb, Embryo. t', Thickening of test over nucleus. m, Mantle. dl, Gill or branchia. 1, Languet. 1-H, Muscle bands of mantle. 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 pro-longed 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 388 usual positions; but in place of any distinct subneural gland there are two lateral neural glandular masses first described by Metcalf
Brooks (25), that the ovary really belongs to the solitary Sal pa, 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 be-longing to another chain, and the advanced in its development before the testis tis far formed. of Snips. 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
placenta
Addendum. The family Octacnemidae includes the single remarkable genus Octacnemus, found during the " Challenger " expedition, and first Oatecne- described by Moseley (29). It is now known in both a mldse. solitary and an aggregated form, and was regarded by Herdman as a deep-sea representative of the pelagic Salpidae, possibly fixed; or, better, as related to the primitive fixed forms from which Salpidae have been derived. Metcalf
A m At A, Solitary form (after Herdman). B, Aggregated form (after Metcalf). a, Anus. m, Mouth. At, Atrial aperture. ce, Oesophagus. br.s, Branchial sac. p.br, Peribranchial cavity. g.s, Gill slit. st, Stolon. 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 216o fathoms, and Metcalf has since described a new species, 0. 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. End of Article: ORDER II If you wish, you can link directly to this article.
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