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 so as to extend for some distance anteriorly, or as is more usual be concentrated 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 two very distinct families, the SALPID.S:, which are the typical members, and the OCTACNEMID.E, including a single very remarkable form (Octacnemus bythius), which in some respects does not conform with the characters given above. The Salpidae includes the single genus Salpa (Forskal), 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. 14, A), in which the aliment ary 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 asex ual (proles solitaria) and the aggregated sexual proles gregaria) which are usually quite unlike one another. The soli tary form (fig. 14, 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 seg mented into a series of or embryos. As,dt gem at A B Fio. 14. Salpa rundnata-fusiformte. A. Aggi-e- buds gated form. B. Solitary form. Lettering as the stolon elongates, the before; 1-9, muscle bands; em, embryo; gem, em v, rvO q npar fi, p f rpp geramiparous stolon; m, mantle; vise, visceral * n ir y? s . ne mass (nucleus). (Original.) end which have become advanced in their deve lopment 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. 15). k Each member of the chain is a Salpa I of the sexual or aggregated form, j and when mature may either still / attached to its neighbours or se- h^ parated from them (fig. 14, A) | produce one or several embryos, which develop into the solitary Salpa. Thus the two forms alterStruc- nate regularly. The more importture of ant points in the structure of a Salpa. typical Salpa are shown in fig. 16. The branchial and atrial apertures are at opposite ends of the body, and each leads into a large cavity, the branchial and peribranchial Fi<>. 15. Posterior part of solitary sacs wriirh arp in frpp rommunira- format Salpa denuxratica-mveron<Ua, ics, wmcn aie in n ica- showing a chain of embryos nearly tion at the sides of the obliquely- ready to be set free, gem, young running dorsal lamina or "gill." aggregated Salpse forming the chain; The test is well developed and ^ stolon; m, muscle band of the i, i, .1 L f /. mantle. (Uncmal.) adheres closely to the surface 01 the mantle. The muscle bands of the mantle do not completely encircle the . em j, body. Theyare 5 present dorsally and laterally, but the major ity do not reach the ventral sur face. In many cases neigh bouring bands join in the med ian dorsal line, Pro 16 _ genii-diagrammatic representation of Salpa from (ng. 14). Ihe leftside. Lettering as before; emb, embryo; m, mantle; anterior end of I, languet; sgd, duct of subneural gland; l-ll, muscle tVio rtnroal la bands of mantle; f, thickening of test over nucleus; dl, the dorsal m or branchia< ( (5 ri gj nal .) nuna is prolonged to form a prominent tentacular organ, the languet, proJ 9,o n end t tn jecting into the branchial sac. The nerve ganglion, subneural gland, dorsal lamina, peripharyngeal bands, and endostyle are placed in the usual positions. A pigment spot and an otocyst are found in connection 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 i.s placed in free communication with the peribranchial cavity, are to be regarded as gigantic stigmata formed by the suppression of tho lateral walls of the branchial sac. Fig. 16 represents an aggre gated 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 (23), 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 Satya. This idea would of course entirely destroy tho view that Salpa is an example of alterna tion 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 Developspermatozoa of an older Salpa belonging to another chain, and ment of the embryo is far advanced in its development before the testis is Salpa. formed. 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 up with oil-globules to form a mass of nutrient material the elseoblast which is used up later on in the develop ment. Many suggestions have been made as to the homology of the elseoblast. The most probable is that it Js the disappearing rudiment of the tail found in the larval condition of most Ascidians. The family Octacnemidse includes the single remarkable form OctaOctacnemus bythius, found during the " Challenger " expedition, and cnemidee. first described by Moseley (29). It is apparently a deepsea representative of the pelagic Salpidse, and may pos sibly be fixed. The body is somewhat discoid, with its margin prolonged tr> fm-m PitrVit taripr Fl - 17. Diagrammatic vertical longitudinal section l i jt? " of Octacnemus bythiits (after Moseley). br, branchial ing^ processes, on to aperture; TO, opening of oesophagus; r, rectum; at, which the muscle atrial aperture; rm, rm, radiating muscles; n, nucleus; bands of the "** musc l es f nucleus; g, respiratory membrane; 6, thickened margin of base of test; pa, pedicle of mantle are con- attachment, tinned. The ali mentary canal forms a compact nucleus (fig. 17); the endostyle is very short; and the dorsal lamina is apparently absent. The re production and life-history are entirely unknown.
Order III. ASCIDIACEA.
Fixed or free-swimming Simple or Compound Ascidians which in Ascidthe adult are never provided with a tail and have no trace of a iacea. 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 per forated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the atrial aperture. Many of the forms reproduce by gemmation, 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.
Sub-order 1. Ascidise Simplices.
Fixed Ascidians which are solitary and very rarely reproduce by Simple gemmation; if colonies are formed, the members are not buried in Ascida common investing mass, but each has a distinct test of its own. ians. 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 Clavelinidss (the Social Ascidians), forms a transition from the typical Simple forms, which never reproduce by gemmation, to the Compound forms, which always do (see p. 618 below). The Ascidise Simplices may be divided into the following families:
Family I. CLAVELINID.E. 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.
