PHYLUM MOLLUSCA 273 



velopmental history of certain mollusks, and many embryolo- 

 gists are inclined to consider this stage an indication of the ances- 

 tral condition. According to this view, the mollusks, annelids, 

 and other animals which pass through a trochophore stage in 

 their ontogeny were all derived from a similar ancestral form. 



