362 



ACTINOTROCHA. 



the mouth 1 . It is at first terminal, but on the development of a 

 large prse-oral lobe it assumes a ventral position. The anus 

 is formed at a later period at the posterior end of the body. 



FlG. 163. A SERIES OF STAGES IN THE DEVELOPMENT OF PHORONIS FROM 



ACTINOTROCHA. (After Metschnikoff.) 



A. Young larva. 



B. Larva after the formation of post-oral ring of tentacles. 



C. Larva with commencing invagination to form the body of Phoronis. 



D. Invagination partially everted. 



E. Invagination completely everted. 



;;/. mouth ; an. anus; iv. invagination to form the body of Phoronis. 



The youngest free larva observed by Metschnikoff (No. 373) 

 was less developed than the oldest larva found by Kowalevsky. 



1 Kowalevsky states that what I have called the mouth is the anus, but his sub- 

 sequent descriptions shew that he has transposed the mouth and anus in the embryo, 

 and that the opening, which he asserts to be the anus, is in reality the mouth. 



