DEVELOPMENT OF PARAVORTEX GEMELLIPARA 461 



Mesostomum ehrenbergi, M. productum, M. lingiia, Bothro- 

 mesostomum personatum, and of one Alloiocoele, Plagiostomiim 

 girardi. 



Another remarkable contribution to Rhabdocoele develop- 

 ment is the section devoted to embryology by P. Hallez in his 

 later work ('08), to which we have already referred, upon Para- 

 vortex cardii, a Rhabdocoele parasitic in the stomach and 

 intestine of the mollusc Cardium edule. He therein reinterprets 

 the significance of certain figures published by Caullery and 

 Mesnil. As a result of the behavior of the ectoderm and ento- 

 derm cells to be described in this paper some of Hallez 's figures 

 in turn suggest an interpretation differing from his own. The 

 condition at corresponding stages in the development of Para- 

 vortex cardii and P. gemellipara shows in many respects a 

 striking similarity. In the formation of the ectoderm and 

 the manner in which the vitellarial yolk enters the embryo, 

 however, the process in P. gemellipara differs distinctly from 

 that described for P. cardii. The ectoderm differentiation 

 agrees essentially with that found by Bresslau in Mesostomum 

 ehrenbergi. On the other hand, though the end result is the 

 same, the manner in which the yolk is taken into the embryo 

 of P. gemellipara presents, so far as the literature studied has 

 revealed, marked variations from that described for any animal. 



While Linton made no serious attempt to discover the origin 

 of the twins in Paravortex gemellipara, he suggests that the 

 twin condition might be due to a process of polyembryony. 

 In order to settle this point J. T. Patterson in the summer of 

 1911 continued the study of the worm at the Woods Hole Labora- 

 tory. The results of his work appeared in 1912. 



The purpose of the present paper is to describe the main 

 features in the behavior and development of Paravortex gemel- 

 lipara. Particular attention is to be paid to the processes of 

 nutrition as taking place in all stages from the oocyte to the 

 free-swimming worm, comparing them with homologous pro- 

 cesses observed by investigators working on other forms. 



It is a pleasure to express here my sincere appreciation of the 

 kind assistance offered me in all stages of this research by Dr. 

 Wesley R. Coe. 



