916 



POLYZOA AND TUNICATA 



any group contains two distinct canals, which send off branches 

 into each peduncle. One of these branches terminates in the heart, 

 which is nothing more than a contractile dilatation of the principal 

 trunk ; this trunk subdivides into vessels (or rather sinuses, which are 

 mere channels not having proper walls of their own), of which some 

 ramify over the respiratory sac, branching off at each of the passages 

 between the oval slits, whilst others are first distributed to the 

 stomach and intestine, and to the soft surface of the mantle. All 



these reunite so as to form 

 a trunk, which passes to the 

 peduncle and constitutes the 

 returning branch. Although 

 the circulation in the dif- 

 ferent bodies is brought 

 into connection by the com- 

 mon stem, yet that of each 

 is independent of the rest, 

 continuing when the current 

 through its own foot -stalk is 

 interrupted by a ligature ; 

 and the stream which re- 

 turns from the branchial 

 sac and the viscera is then 

 poured into the posterior 

 part of the heart instead 

 of entering the peduncle. 



The development of the 

 Ascidians, the early stages 

 of which are observable 

 whilst the ova are still 

 within the cloaca of the 

 parent, presents some phe- 

 nomena of much interest 



*, p.bi: 



to the microscopist which 

 alone can be noticed here. 

 After the ordinary repeated 

 segmentation of the yolk, 

 whereby a ' mulberry mass ' 

 is produced, a sort of ring 



FIG. 692. Diagrammatic longitudinal section of 

 Ascidia showing the heart, the blood-vessels, 

 the branchial sac, the alimentary canal, &c. 

 from the left side : br.si., branchial siphon : 

 at.si., atrial siphon ; t., test ; m., mantle ; 

 br.s., branchial sac; p.br., peribranchial 

 cavity; cl., cloaca; n.g , nerve ganglion; 

 tn., tentacle; gl., neural gland; ce.a., ceso- 

 phageal aperture ; st., stomach ; i., intestine ; 

 r., rectum; a., anus; o.w., genital organs; 



g.d., genital ducts; h., heart; c.sp., cardio- pnoirplino- its rpntrnl 



splanchnic vessel; v.t., vessel to the test; 

 t.k., terminal knob on vessel in test; v.t'., 

 vessel from the test; v.st., vessel to the 

 stomach &c. ; v.m., vessel to the mantle; 

 v.m!., vessel from the mantle ; d.v., dorsal 

 vessel ; tr., transverse vessel of branchial 

 sac ; l.v ., fine longitudinal vessel of branchial 

 sac ; sg., stigmata of branchial sac ; v.v., 

 ventral vessel ; br.c., branchio-cardiac 

 vessel; sp.br., splanchno-branchial vessel. 

 (After Prof. Herdman.) 



portion ; but this soon 

 shows itself as a tapering 

 tail-like prolongation from 

 one side of the yolk, which 

 gradually becomes more 

 and more detached from 

 it, save at the part from 

 which it springs. Either 



whilst the egg is still within 

 the cloaca, or soon after it has escaped from the vent, its envelope 

 bursts, and the larva escapes, and in this condition it presents very 



