68 CONKLIX — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



Hydra, or the tailed larva of Botryllus is an Appendicularia, and 

 more so than a tadpole is an urodellan Batrachian." This close 

 relationship he bases largely upon the external resemblances be- 

 tween the larvae of Thecidium and various Polyzoon larvae. It 

 seems to me that some of these resemblances are real homologies, 

 but on the other hand the differences between these larvae, as well 

 as between the adults of these two groups, are so great that it 

 would be inadvisable to place them together in the same class ; 

 though I believe they should be placed in the same phylum. 

 Moreover it seems to me that Brooks' view, that the Polyzoa are 

 the ancestral form of which the Brachiopoda are a specialization, 

 is just the reverse of the real relationship ; larval as well as adult 

 Brachiopoda show less specialization and certainly less degeneration 

 than the Polyzoa. 



The resemblances of the brachiopod larva to the MoUuscan 

 veliger, upon which Brooks lays emphasis, are in the main the same 

 as the resemblances to the trochophore, the veliger and trochophore 

 belonging to the same type of larva. 



The idea that the brachiopods are related to the chsetognaths, 

 which was suggested by Blitschli and Hertwig ('80) and maintained 

 by van Bemmelen ('83), has little more in its favor than the sup- 

 posed resemblance in the method of formation of the coelom and 

 in certain histological details. 



So far as the formation of the coelom is concerned, I have 

 already pointed out the fact that in Terebratulina it forms in a very 

 •different manner from what obtains in Sagitta, and as for the histo- 

 logical resemblances they are by no means confined to the two 

 groups in question. On the other hand there are so many im- 

 portant differences between the two groups, both in their embryology 

 and in their adult structure, that one could as well maintain the 

 •affinity of the Brachiopoda with Echinodermata, Enteropneusta or 

 Chordata, as with Chsetognatha. 



Caldwell ('82) first pointed out in detail the resemblances be- 

 tween Fhoronis and the Brachiopoda. In this paper he has urged 

 *'an entirely new view of the homologies of the body surfaces in 

 Brachiopoda." He regards the Brachiopoda as fixed by their ven- 

 tral surface, and both valves of the shell as ventral in position, the 

 peduncle of the brachiopod corresponding to the ventral invagina- 

 tion of Actinotrocha. While there are some facts which may be 

 urged in favor of this view there are many which may be used 



