452 TUNICATA. 



the body. Similar conditions are found in the proliferating stolon of 

 other Tunicates (e.g., Clavelina, p. 370). In this form also, in the 

 stolon, a dorsal blood-current is separated from a ventral current 

 flowing in the opposite direction by the epicardial sac which forms a 

 partition extending almost to the end of the stolon (Fig. 229, x). A 

 transverse section through the proliferating stolon of Clavelina shows, 

 on the whole, remarkable agreement with one through the post- 

 abdomen of the Polyclinidae, though important variation is found in 

 the position of the heart. In the Polyclinidae, the heart lies at the 

 posterior distal end of the post-abdomen (Figs. 226 and 227), which, 

 consequently, contains important organs belonging to the organisa- 

 tion of the parent (the heart and the genital organs). In Clavelina, 

 the pericardial vesicle and the heart have shifted to the point of 

 origin of the stolon (Fig. 173 C, p. 375). The heart lies at the 

 proximal end of the epicardial sac, on the ventral side of the latter, 

 so that the epicardial lamella, as shown above (p. 370), can be utilised 

 in the formation of the dorsal wall of the heart. The proliferating 

 stolon of Clavelina does not contain any important organ of the 

 parent body, but is a process of the body dedicated exclusively to a 

 sexual reproduction. The same is the case with the stolon of other 

 Tunicates (Thaliacea and Pyrosoma, etc.). We might imagine the pro- 

 liferating stolon as derived from the post-abdomen of the Polyclinidae, 

 if we chose to assume that the genital organs and the heart withdrew 

 to the proximal end of the post-abdomen, which then became dedicated 

 exclusively to the function of reproduction. Such an assumption 

 would satisfactorily explain the remarkable divergence in the position 

 of the heart above alluded to. While, in Clavelina, the heart lies on 

 the ventral side of the epicardial sac, in Pyrosoma it is found on the 

 dorsal side of the so-called endostyle- process (Fig. 253, hz, p. 485). 

 If, then, we assume that in these two groups the heart originally lay, 

 as in the Polyclinidae, at the distal end of the stolon, it is not difficult 

 to imagine that secondary shifting took place, in the Clavelina to the 

 ventral and in Pyrosoma to the dorsal side. 



The ontogeny of the Pyrosoma would, indeed, rather lead us to consider 

 the change of position of the heart as the consequence of lateral shifting of 

 the organs. The heart there lies originally on the right side of the entoderm- 

 process, and only later shifts to the dorsal side (SEELIGER) ; cf. p. 493. 



It is evident from the above that the asexual reproduction of the 

 Polydinidae (through the segmentation of the post-abdomen) and 

 the stolonic budding found, for instance, in Clavelina, are related one 

 to the other. GIABD (No. 57) has already pointed out that the 



