iij^ John H. Gerould, 



blances to the adult PJwronis and to the adult Bryozoa. These 

 resemblances consist chiefly in the appearance of the crown of 

 tentacles, in the position of the anus, the unsegmented body, and 

 in the presence of one or two pairs of nephridia. It is on the 

 basis of these characteristics that Lang constructed the group 

 Prosopygii. 



But embryology shows that the crown of tentacles of the Sipun- 

 culids, which are outgrowths of the preoral lobe, are in no wise 

 homologous with the lophophore of Bryozoa, or with the tentacles 

 of Phoronis. In the latter, both the larval and the definitive ten- 

 tacles, which grow out behind them, are postoral in position, and 

 the epistome represents the prostomium. In the former the extra- 

 ordinary character of the metamorphosis makes it difficult ta 

 establish any homologies whatever, but the epistome appears to 

 correspond to that of PJwronis, and the tentacles to the larval,, 

 deciduous, tentacles of that form. The position of the anus in the 

 Bryozoa is probably dorsal as in Phoronis, but this feature has 

 little value in indicating the affinities of the Sipunculids, for the 

 Priapulids, which Lang considers a the nearest allies of the Sipun- 

 culids, have a terminal anus. Finally, I have already pointed out 

 that there is not in the Sipunculids a complete lack of segmentation. 



The strongest affinities of the Sipunculids, as shown by their 

 embryology, are with the Annelids, rather than with these aberrant 

 Vermideans. The trochophore of Phascolosoma is essentially iden- 

 tical with that of a mesotrochal Annelid, and I am even inclined 

 to think that a closer relationship exists between the Sipunculids 

 and the primitive Molluscs, such as Chaetoderma, than with the 

 Vermidean groups represented by Phoronis, the Bryozoa and the 

 Brachiopods. 



E. Meyer (1904), in a recent speculative inquiry into the origin 

 of the Echinoderms, seeks to show that the hydrocoel in that phylum 

 has arisen from the anterior body cavity of a terebelloid ancestor 

 with resemblances to the "Prosopygii" of to-day. 



In the Terehellidae Meyer's own observations show that there 

 is an anterior body cavity, consisting of the united cavities of the 

 first five segments, separated by a diaphragm (septum) from a 

 posterior chamber. In brief, the septa in fiont of the diaphragm, 

 and throughout the greater part of the region behind it, have been 

 suppressed, and a pair of sacs containing diverticula of the anterior 

 cavity extend backward from the sides of the diaphragm. The 



