J26 John H. Gerould, 



the same species, with 30—40 similar hooks would tend somewhat 

 to corroborate his other observation. 



Finally Selenka states a tentative opinion that Phascolosoma 

 is closely related to the Chaetopods on account (1) of the persistance 

 of the egg- membrane as the larval cuticula, (2) the possession of a 

 hollow prostomium, (3) the presence of lateral bristles, (4) the 

 peculiarities of the cleavage, (5) the presence of circlets of cilia and 

 (6) the origin of the nerve cord from a thickening of the ectoderm. 

 I have shown that the first two statements rest probably on errors 

 of observation, and that the third does not apply to three other 

 species of this genus. Nevertheless, the occurrence of such paired 

 bristles, the method of cleavage, the presence of circlets of cilia 

 and the origin of the nerve cord still claim our attention as pointing 

 toward the relationships of Phascolosoma, and will be considered in 

 the following section. 



11. Comparisons and Conclusions. 



Comparison with Sipunculus. 



These studies have show^n not only greater resemblances in the 

 developmental processes of Phascolosoma and Sipunciihis than appeared 

 from the investigations of Selenka (1875) and Hatschek (1883), but 

 have thrown a flood of light upon the hitherto little understood 

 embryonal envelop of Siimnculus. In an earlier paper (1903) I have 

 shown that the serosa is simply a highly modified prototroch. The 

 similarity between the serosal or prototrochal cells in the younger 

 embryos of Sipuncidus (Hatschek, 1883, fig. 8—18) and the corre- 

 sponding cells in Phascolosoma (PI. 35 of this paper) is evident. 

 These ciliated cells adhere to the zona radiata in Sipunculus, but 

 the rest of the ectoderm sinks away from it, with the exception of 

 the apical rosette of the larva, thus forming the amniotic cavnty 

 of the head, that of the trunk, and the intervening amniotic canal, 

 which passes through the dorsal interruption of the prototroch. 



I have shown that the zona radiata in Phascolosoma is cast off, 

 a new cuticula previously having been secreted beneath it over the 

 entire ectoderm ; that the cytoplasm of the prototroch cells, enclosed 

 by this cuticula is transformed ' into yolk granules, which are forced 

 backward into the coelom, whereas in Sipunculus the prototroch cells 

 (serosa) are cast off with the zona radiata to Avhich they adhere. 



