ASCIDIACEA DEVELOPMENT OF THE FREE-SWIMMING LARVA. 369 



ventral wall of the alimentary canal near the point at which the 

 oesophagus enters the pharynx. The origin of these <3ell -strands 

 (in-m-.n-ilitnn) is less clearly stated. VAN BENEDEN and JULIN have 

 no doubt that they become detached from the entodermal intestinal 

 epithelium. WILLEY has recently (No. 54a) made the same observa- 

 tion, finding, however, that the procardial rudiment is unpaired and 

 also not entirely agreeing with VAN BENEDEN and JULIN with regard 

 to its further development. The left procardial strand always appears 

 stronger than the right. The strands soon develop lumina and thus 

 become tubes. In later stages, the posterior ends of these tubes fuse, 

 while, anteriorly, they open into the branchial sac. The whole rudi- 



l"i<;. 171. Three consecutive transverse sections through the trunk-region of a Clav-e 

 lina larva (diagrammatic, after VAN BENEDEN and JULIN). A , shows the posterior 

 blind end of the branchial sac or pharynx with the apertures of the oesophagus and 

 the epicardial tubes (ep.o). B, shows the connection between the epicardial tubes 

 (ep) and the pericardial vesicles (pc). C, shows the blind ends of the epicardial 

 tubes (t'ji) and the pericaridal vesicle severed from them (pc). ec, ectoderm! ep, 

 epicardial tubes; ep', blind ends of the epicardial tubes ; ep.o, the epicardial tubes 

 near their openings into the pharynx ; h, heart ; m, cellulose mantle ; n, neural 

 tube ; oe, oesophagus ; p, peribranchial sac; pc, pericardial cavity. 



incut now consists of ;tn unpaired posterior caecum (Fig. 171 B, ep 

 and pc), which forks anteriorly into two tubes that open separately 

 into the branchial sac (Fig. 171 A, ep.o). From the posterior 

 caecum, a vesicle becomes abstricted (Fig. 171 C, pc) and this 

 represents the common rudiment of the heart and the pericardium. 

 The lumen of this vesicle (pc) is the future pericardial cavity. 

 The form of the vesicle is complicated in consequence of the invagina- 

 tion of its dorsal wall as a furrow running along its whole length ; 

 this makes the vesicle crescent-shaped in cross -section. The lumen 

 of this invagination is the future cavity of the heart (h). The in- 



BB 



