448 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULIDjE. 



lids undergo a degeneration similar in some respects at least to that in Phascolosoma. 

 Thus Mead ('97, p. 261) states that in Amphitrite "The prototroch and paratroch 

 before their actual disappearance undergo a marked degeneration. The cells shrink 

 and become filled with yellow granules." The process of dissolution of the proto- 

 troch and the replacement of it by definitive ectoderm has not been described in 

 Amphitrite, but aside from the question whether its substance is gradually absorbed 

 in situ, as appears to be the case, or passes in the form of visible yolk-granules into 

 the ccelom as in Phascolosoma, it is clear that there is in respect to the prototroch 

 a remarkable similarity between the two forms. 



Shedding of the prototroch, moreover, occurs both in the annelids and the mol- 

 lusks. Eisig ('98, pp. 81, 108) describes the degeneration and casting off of the 

 peripheral part of the prototroch in Capitella, and suggests that it may perhaps occur 

 normally in Polygordius, in which Hatschek has observed that groups of ciliated pro- 

 totroch cells, undergoing degeneration, are sloughed off. Hatschek, however, regards 

 this as a pathological process and maintains ('78, p. 50) that the prototroch cells hi 

 Polygordius gradually diminish in height and assume the characters of other epithe- 

 lial cells. 



Meisenheimer (:01) sets forth the general homology between the velum of 

 Dreissensia and the prototroch of annelids, which is evident if the former term be 

 restricted to the two posterior rows of cells of the velum, so as to exclude the apical 

 plate and " Dach des Velums" of Dreissensia. The vacuolization and flattening of 

 the cells of the prototroch proper and the attenuation of those which form the roof 

 of the velum, all of which are finally cast off, furnish some interesting points of simi- 

 larity in the fate of the prototroch in a mollusk and in a sipunculid. 



I have endeavored to show that the serosa of Sipunculus represents the remains 

 of a degenerating prototroch equivalent to that of Phascolosoma, which in turn is 

 homologous to the prototroch of mesotrochal annelids. Which of the three types 

 represents the most primitive condition? Clearly it is the prototroch of the annelids. 

 Waiving for the present the question as to whether the sipunculids have been derived 

 from segmented or unsegmented ancestors, there can be little doubt that they sprang 

 from forms in which the prototroch was, like that of the annelids, of moderate size 

 and without the specially acquired functions of protection and nutrition which it 

 performs in the sipunculids. 



It is quite conceivable that the differences between the prototroch in Phasco- 

 losoma and in Sipunculus may have arisen as an effect of the presence or absence 

 of yolk. The adaptation of form and habit to various quantities of yolk is evident 

 even in the different species of Phascolosoma. Thus in P. gouldii there is a compara- 



