STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULIDjE. 447 



V. FATE OF THE PROTOTROCH IN THE SIPUNCULIDS. 



From the foregoing account it is evident that there is an essential similarity 

 between the prototroch of Phascolosoma and the serosa of Sipunculus. This simi- 

 larity holds not only in the position and probable number of the cells and the arrange- 

 ment of these between correspondingly apical and somatic regions of ectoderm, but 

 also in the transitory nature of each structure and their common function. The 

 cells which I regard as the prototroch in Sipunculus spread backward past the margin 

 of the somatic plate, and form a complete embryonal envelope or serosa in which 

 nuclei can no longer be seen; the serosa dwindles into an exceedingly thin layer, 

 and its substance, according to Hatschek, is probably absorbed by the embryo. 

 The remnant is finally cast off with the zona radiata. 



Its homologue in Phascolosoma, on the other hand, appears as a typical pro- 

 totroch of relatively huge size, which likewise becomes flattened out against the zona 

 radiata, and covers a broad equatorial region of the trochophore; but it never forms 

 an embryonal envelope, and at the time of the shedding of the zona radiata it is not 

 cast off with this structure, but disintegrates and passes into the body-cavity. 

 l-\l Thus in Sipunculus probably, and in Phascolosoma surely, it is a nutritive organ. 

 In Phascolosoma the cytoplasm of each prototroch cell becomes converted, before its 

 final disintegration, into yolk-granules which, passing into the ccelom, completely fill 

 the ccelomic fluid (Fig. 16) and form the chief source of nourishment for the larva 

 during the first week. At the end of this period most of the yolk has been absorbed. 



VI. PHYLOGENETIC SIGNIFICANCE OF THE PROTOTROCH. 



Since no embryonal envelope was apparent in Phascolosoma, according to 

 Selenka's brief account of the development, Hatschek raised the question as to whether 

 the serosa of Sipunculus had been acquired during a comparatively short phylo- 

 genetic period from conditions like those in Phascolosoma without a serosa or whether, 

 on the other hand, there had been an atrophy and loss of the structure in the latter 

 form. We are now in a position to answer this question with some degree of certainty, 

 or at least to form some definite opinions in regard to the matter, which is all that 

 embryological evidence alone can enable us to do with questions of phylogeny. 



The prototroch of Phascolosoma resembles in many respects that of annelids. 

 The large primary prototroch cells of the former correspond precisely in origin to 

 those of the annelids. There is evidence also that the prototroch cells of anne- 



