The Development of Phascolosoma. \^\ 



no similarity, in view of the fact that introversion of the prostomium 

 takes place in the Sipunculids, but not in Echiurids. 



(3) The anus in Sipimculus and Phascolosoma arises from the 

 middle of the dorsal side of the trunk. In this respect Sipunculids 

 resemble Etipomatns more than they do Echiurids. 



(4, 5) Nothing- homologous to the anal vesicles has been demon- 

 strated in Sipunculids, nor have head kidneys been found. 



(6, 7) The next two points of difference, that have been urged 

 by earlier writers, have been made less emphatic by my studies on 

 Fhascolosoma. The young- larva of Phascolosoma has a distinct 

 prostomium. A still more prominent preoral lobe occurs in a Sipun- 

 €ulid larva, probably of Phymosoma, which I studied at Naples. 

 Selenka found a similar structure in the southern form of Ph. 

 ehngatum, which he perhaps erroneously described as hollow. The 

 elongated prostomium of the Echiurid, however, appears to have no 

 homologue in the adult Sipunculid. Its position is occoupied in 

 Phascolosoma by a ciliated sense organ. 



Finally, the signs of segmentation in the trochophore of Phasco- 

 losoma are so much less prominent than the metamerism of the Echi- 

 urids. as expressed in the paired lateral nerves of the larva and in 

 the several pairs of nephridia in the adult, as to constitute an 

 additional reason for separating these two groups. 



The Echiurid is, in my opinion, a degenerate Chaetopod in which 

 an enormously elongated prostomium has developed, carrying with 

 it the supraoesophageal ganglion and gradually lengthening connec- 

 tives. ToKEEY (1903), whose recent studies on the cell lineage of 

 Thalassema mellita give an excellent basis for compaiison of the 

 Echiurids with other forms, regards them, however, as possibly 

 primitive, citing as evidence in favor of this view the meagre de- 

 velopment of the coelomesoblast, the formation of the ectomesoblast 

 from the first quartet, the presence of excretory cells, and the late 

 formation of the anus (p. 217). Toreey's work shows that a sur- 

 prising similarity exists between the development of Thalassema 

 and that of PodarJce, a typical Chaetopod with equal cleavage de- 

 scribed by TßEADWELL (1901), and furthermore that the former pre- 

 sents no distinctly sipunculoid features. 



Comparisons with Molluscs. 



The development of the Sipunculids presents nearly as many 



points of resemblance to the Molluscs as to the Annelids. This is 



9* 



