396 



TUNICATA. 



The transformations undergone, hi the further course of develop- 

 ment, by the paired coelomic sacs, the lumina of which had become 



FIG. 191. Three germ-discs of Pyrosoma, diagrammatic (after SALENSKY). d, enteric 

 cavity ; es, endostyle ; Ic, left coelomic sac ; n, rudiment of the nervous system ; 

 p, peribranchial tubes ; pc, pericardial sac = re, right coelomic sac. 



connected posteriorly, are of importance. Only the right coelomic 

 sac is retained (Fig. 191, re), while the left* undergoes degeneration 

 (Fig. 191 A-C), its lumen becoming smaller and its cells losing their 



FIG. 192. Two embryos of Pyrosoma (after KOWALEVSKY). cl, atrial aperture of the 

 Cyathozooid. en, endostyle-folds ; n, nervous system ; <>, aperture of one of the peri- 

 branchial tubes ; p, peribranchial tube ; pc, pericardial sac ; pc', posterior tubular 

 continuation of the pericardial sac ; x, posterior part of the germ-disc raised up from 

 the surface of the egg (rudiment of the stolon)'; z, cell-zone. 



* The terms " right " and " left " refer to the arrangement of the organs in 

 the adult Cyathozooid, in which the atrial aperture denotes the posterior end 

 of the body. As, in our orientation of the germ-disc, the atrial aperture lies 

 at the anterior edge of the disc, the right and left organs appear to be reversed. 

 Our orientation of the germ-disc is, however, intentional (p. 404), having been 

 retained, in accordance with the views of authors, since the opposite orienta- 

 tion would also lead to certain difficulties in describing the processes (especially 

 in connection with the development of the Ascidiozooid). 



