192 



TUNICATA 



Struc- 

 ture of 

 Salpa. 



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^E, 

 which are the typical members, and the OCTACNKMID.S, including a 

 single very remarkable form (Octacnemus lylhius), which in some 

 respects does not conform with the characters given above. 



The Salpidas 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 

 end - EH31- 3 ne another. The soli- 

 tary form (fig. 14, B) 

 gives rise by internal 

 d I 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 

 Fio. 14. Salpa nmcinata-fusiformis. A. Aggre. buds or embryos. As 

 gated form. B. Solitary form. Lettering as the stolon elongates, the 

 before ; 1-9, muscle bands ; em, embryo ; gem, embryos near the f ree 

 geramiparous stolon ; in, mantle ; vise, visceral , > . . . , 

 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). ' > 

 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 se- 

 parated from them (fig. 14, A) 

 produce one or several embryos, 

 which develop into the solitary 

 Salpa. Thus the two forms alter- 

 nate regularly. The more import- 

 ant points in the structure of a 

 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 peribrauchial Fio. 15. Posterior part of solitary 

 sacs, which are in free communiea- gi^owinga ^hain'of < efinbiyo*nearly 

 tiou at the sides of the obliquely- ready to be set free, gem, young 

 runnin" dorsal lamina or "gill." aggregated Salpie forming the chain ; 

 The test is well developed and s, stolon; m, muscle band of the 

 adheres closely to the surface of mantle ' < 0rl g' nal -> 

 the mantle. The muscle bands of the mantle do not completely 



I'ISC 



8 

 -9 



-V gem 



em b 



br 



encircle the 

 body. They are 

 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, Fia _ lg> _ g e mi-diagrammatic representation of Salpa from 

 (fig. 14). The leftside. Lettering as before ; emb, embryo ; m, mantle ; 

 anterior end of I, languet; sgd, duct of subneural gland; 1-11, muscle 

 tlin rlnrsil la bands of mantle ; f, thickening of test over nucleus ; ill, 



. UUI . MJ gin or branchia. (Original.) 



imna is pro- 



longed to form a prominent tentacular organ, the languet, pro- 



Cild' 



jccting 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 largo 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. 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 (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 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 Devek 

 spermatozoa of an older Salpa belonging to another chain, and ment ( 

 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-globnles to form a mass of nutrient 

 material the elaeoblast 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 is the disappearing 

 rudiment of the tail found in the larval condition of most Ascidians. 



The family Octacnemidee includes the single remarkable form Octa- 

 Odacnemus bythius, found during the " Challenger " expedition, and cnemic 

 first described by 

 Moseley (29). It is 

 apparently a deep- 

 sea representative 

 of the pelagic Sal- 

 pidse, and may pos- 

 sibly be fixed. The 

 body is somewhat 

 discoid, with its 

 margin prolonged 



tnfnrm picrhttniipr Fl - 17. Diagrammatic vertical longitudinal section 

 ' of Octamemm lythtus (after Moseley). br, branchial 

 ing_ processes, onto aperture; m, opening of oesophagus ; r, rectum; at, 

 which the muscle atrial aperture ; rm, rm, radiating muscles ; n, nucleus ; 

 bands of the 

 mantle are con- 

 tinued. 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 Ascid- 

 the 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 Pyrosoina. 



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 Ascid- 

 a 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 ClavclinidsR (the Social Ascidians), forms a transition 

 from the typical Simple forms, which never rejiroduce 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. 



mh, muscles of nucleus; g, respiratory membrane; 

 6, thickened margin of base of test ; pa, pedicle of 

 attachment. 



