ASCIDIACEA TRANSVERSE FISSION. 



451 



is also evidently the homologue of the entodermal or endostyle-process 

 in the proliferating stolon of the Salpidae and Pyrosoma, but the rela- 

 tive position of the heart distinguishes this so-called epicardial tube 

 in the Polyclinidae from 

 the similar tube in the 

 above-mentioned forms- 

 The finer anatomical 

 features of this tube have 

 been described by MAU- 

 RICE (No. 40) in Frtiij'i- 

 roides. The tube forks at 

 its anterior end ; the two 

 prongs of this fork have 

 been distinguished by VAN 

 BENEDEN and JULIN as 

 epicardial tubes from the 

 posterior undivided epi- 

 cardial sac (see above, p. 

 370). The two epicardial 

 tubes arise on either side 

 of the median line behind 

 the posterior end of the 

 endostyle from the pharynx. The posterior end of the sac is, according 

 to MAURICE, also forked (Fig. 226 A and D, ep). It embraces the 

 crescent-shaped pericardial vesicle (p). In Clavelina, the epicardial 

 tube enters into close relation to the heart (p. 370), completing 

 the dorsal wall of that organ, but this is not the case, according to 

 MAURICE, in the Polyclinidae. The heart-tube (h) in these latter is 

 an in vagina t ion of the outer wall of the U-shaped pericardial vesicle 

 (Fig. 226 C and D). 



It should be mentioned that the paired apertures of the two epicardial tubes 

 can be recognised only in larvae and in -quite young asexually-produced 

 individuals. They could not be found by KOWALEVSKY in the adult zooid, 

 and MAURICE also has recently stated that the two epicardial tubes, although 

 they approach close to the wall of the branchial sac, seem no longer to com- 

 municate with its cavity. 



The epicardial sac in the Polyclinidae divides the body-cavity into 

 a dorsal and a ventral half (Fig. 225, 6, 6'). Through each of these 

 halves a blood-stream flows, but, in the dorsal half, the direction of 

 the stream is opposed to that in the ventral half, the blood flowing 

 in the one half towards the heart and in the other from the heart to 



FIG. 226. Diagrams illustrating the condition of 

 the posterior end of the post-abdomen in Fraga- 

 roides (after MAURICE). A, side view ; B, trans- 

 verse section at the level a-b ; C, at the level c-d ; 

 />, at the level e-f. ep, epicardial sac ; h, heart ; 

 P, pericardial sac ; x, forked end of the epi- 

 cardial sac. 



