Page:EB1911 - Volume 27.djvu/405

 usual positions; but in place of any distinct subneural gland there are two lateral neural glandular masses first described by Metcalf. These have no connexion with the ciliated funnel, but open by lateral ducts into the branchial cavity. Median and lateral eyes are also found in connexion with the ganglion. The large spaces at the sides of the dorsal lamina (often called the gill or branchia of Salpa), by means of which the cavity of the branchial sac is placed in free communication with the peribranchial cavity, are to be regarded as gigantic stigmata formed by the suppression of the lateral walls of the branchial sac. Fig. 23 represents an aggregated or sexual Salpa which was once a member of a chain, since it shows a testis and a developing embryo. The ova (always few in number, usually only one) appear at a very early period in the developing chain Salpa, while it is still a part of the gemmiparous stolon in the body of the solitary Salpa. This gave rise to the view put forward by Brooks (25), that the ovary really belongs to the solitary Salpa, 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

belonging to another chain, and the embryo is far advanced in its development before the testis is formed. 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 of the cavity in which it lies, to form the “placenta,” in which the embryonic and the maternal blood streams circulate in close proximity (or actually coalesce during one period) and so allow of the passage of nutriment to the developing embryo. At a somewhat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled with oil-globules to form a mass of nutrient material—the elaeoblast—which is used up later on in the development. Many suggestions have been made as to the homology of the elaeoblast. The most probable is that it is the disappearing rudiment of the tail found in the larval condition of most Ascidians.

The family Octacnemidae includes the single remarkable genus Octacnemus, found during the “Challenger” expedition, and first described by Moseley (29). It is now known in both a 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, however, has shown that the aggregated form of O. patagoniensis, which he has described, is more nearly related to the Clavelinidae amongst Ascidiacea. The body is somewhat discoid, with its margin prolonged to form eight tapering processes (fig. 24), on to which the muscle bands of the mantle are continued. The alimentary canal 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 2160 fathoms, and Metcalf has since described a new species, O. 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.

Fixed or free-swimming simple or compound Ascidians which in the adult are never provided with a tail and have no trace of a notochord. The free-swimming forms are colonies, the simple Ascidians being always fixed. The test is permanent and well developed; as a rule it increases with the age of the

individual. The branchial sac is large and well developed. Its walls are perforated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the atrial aperture. Many of the forms reproduce by gem in m ation, and in most of them the sexually-produced embryo develops into a tailed larva.

The Ascidiacea includes three groups—the simple Ascidians, the compound Ascidians and the free-swimming colonial Pyrosoma.

Fixed Ascidians which are solitary and very rarely reproduce by gemmation; if colonies are formed, the members are not buried in a common investing mass, but each has a distinct test of its own. No strict line of demarcation can be drawn between the simple and the compound Ascidians, and

one of the families of the former group, the Clavelinidae (the social Ascidians), forms a transition from the typical simple forms, which never reproduce by gemmation, to the compound forms, which always do. The Ascidiae Simplices may be divided into the following families:—

Family I., Clavelinidae.—Simple Ascidians which reproduce by gemmation to form small colonies in which each ascidiozooid has a distinct test, but all are connected by a common blood system, and by prolongations of “epicardiac tubes” from the branchial sacs. Buds formed on stolons which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm and endoderm of the ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. This family contains, amongst others, the following three genera: Ecteinascidia (Herdman), with internal longitudinal bars in branchial sac; Clavelina (Savigny), with intestine extending behind branchial sac; and Perophora (Wiegmann), with intestine alongside branchial sac.

Family II., Ascidiidae.—Solitary fixed Ascidians with gelatinous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed. Branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple. This family is divided into three sections:—

Sub-family 1, Hypobythinae.—Branchial sac with no internal longitudinal bars. One genus, Hypobythius (Moseley).

Sub-family 2, Ascidinae.—Stigmata straight. Many genera, of which the following are the more important: Ciona (Fleming), dorsal languets present; Ascidia (Linnaeus, = Phallusia, Savigny), dorsal lamina present (see figs. 1 to 10); Rhodosoma (Ehrenberg), anterior part of test modified to form operculum; Abyssascidia (Herdman), intestine on right side of branchial sac.

Sub-family 3, Corellinae.—Stigmata curved. Three chief genera: Corella (Alder and Hancock), test gelatinous, body sessile; Corynascidia (Herdman), test gelatinous, body pedunculated; Chelyosoma (Brod. and Sow.), test modified into horny plates.

Family III., Cynthiidae.—Solitary fixed Ascidians, usually with leathery test; branchial and atrial apertures both four-lobed. Branchial sac longitudinally folded (fig. 26); stigmata straight; tentacles simple or compound. This family is divided into three sections:—

Sub-family 1, Styelinae.—Not more than four folds on each side of branchial sac (fig. 26, S) tentacles simple. The more important genera are: Styela (Macleay), stigmata normal, and Bathyoncus (Herdman), stigmata absent or modified.