394 



TUNIC ATA. 



orms the vesicular rudiment of the ganglion. In cross-sections this 

 anterior part of the germ-disc is seen to be bilaminar (Fig. 186 B} r 

 as the mesoderm of the germ-disc does not extend so far forward. 



A 



B 





FIG. 187.- Two germ-discs of Pyrosoma (after KOWALEVSKY). n, rudiment of the 

 nervous system ; o, aperture of one of the peribranchial tubes ; p, peribranchial 

 cavity (tube). 



The two peribranchial sacs or tubes appear as ectodermal invagina- 

 tions (Fig. 187 A, p) directed from before backward, which soon 

 lengthen (Fig. 187 B) and show, at the anterior end, the original 



aperture of invagination (o). 

 The two anterior ends with 

 their apertures unite in 

 front of the rudiment of 

 the nervous system (n) 

 (KOWALEVSKY), and thus 

 give rise to the atrial 

 aperture (Fig. 189, d) of 

 the Cyathozooid. Accord- 

 ing to SALENSKY, on the 

 contrary, the latter is pro- 

 duced by an unpaired ecto- 

 dermal invagination with 

 which the anterior ends of 

 the peribranchial tubes 

 come into contact, the 

 original apertures of these 

 tubes having closed before 

 the formation of the cloaca 



FIG. 188. Transverse sections through the germ- 

 disc of Pyrosoma, at the stage depicted in 

 Fig. 187 .-I* (after SALENSKY). A, through the 

 anterior part of the disc, with the rudiment of 

 the nervous system ; jB, through the middle part 

 with the peribranchial sacs, dh, enteric cavity ; 

 dz, yolk-cells ; ec, ectoderm ; en, entoderrn ; n, 

 rudiment of the nervous system; p, invagina- 

 tions of the peribranchial sacs. 



(SALENSKY). 



