Page:EB1911 - Volume 27.djvu/404

Rh 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

characterized by the possession of nine muscle bands, an auditory sac on the left side of the body, a ventrally-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 migrate from their place of origin on the stolon, divide by fission, and become attached to the dorsal outgrowth of the body of the nurse, where they develop. The three forms produced are as follows. (1) Nutritive forms (trophozooids), which remain permanently attached to the nurse and serve to provide it with food; they have the body elongated dorso-ventrally, and the musculature is very slightly developed. (2) Foster forms (phorozooids), which, like the preceding, do not become sexually mature, but, unlike them, are set free as cask-shaped bodies with eight muscle bands and a ventral outgrowth, which is formed of the stalk by which the body was formerly united to the nurse. On this outgrowth the (3) forms (gonozooids) which become sexually mature are attached while still young buds, and after the foster forms are set free these reproductive forms gradually attain their complete development and are eventually set free and lose all trace of their connexion with the foster forms. They resemble the foster forms in having a cask-shaped body with eight muscle bands, but differ in having no outgrowth or process, and in having the reproductive organs fully developed.

Anchinia, of which only one species is known, A. rubra, from the Mediterranean, has the sexual forms permanently attached to

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).

Free-swimming pelagic forms which exhibit alternation of generations in their life-history and in the sexual condition form colonies.

The body is more or less fusiform, with the long axis antero-posterior, and the branchial and atrial apertures 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, or—as is more usual—be 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, including the single

genus Salpa (Forskål), which, however, may be divided into two well-marked groups of species—(1) those, such as S. pinnata, in which the alimentary canal is stretched out along the

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 altogether are known; they are all pelagic forms and are found in nearly all seas. Each species occurs in two forms—the 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 diverticulum from the mantle so as to form “chains” (fig. 22). Each member of the chain is a Salpa of the sexual or aggregated form, and when mature may—either still attached to its neighbours or separated from them (fig. 21, A)—produce one or several embryos, which develop into the solitary Salpa. Thus the two forms 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, the branchial and peribranchial sacs, which are in free 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 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 prolonged 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