70 PRIMITIVE ANIMALS 



especial interest that a similar larval form is assumed 

 by some Molluscs, which in the adult state are so 

 entirely different in their whole organization from 

 worms of any description. This fact implies that the 

 Trochosphere is an exceedingly ancient type of larva, 

 which must have been possessed at some period by 

 the common ancestor of the Annelids and Molluscs, 

 and that this period is a very remote one is shown by 

 the fact that the adult Annelid and Mollusc at the 

 present time have nothing in common that would lead 

 us to suppose that they are in any way connected. 

 There is however nothing to support the view that the 

 Trochosphere in any way represents the common 

 ancestor of the Annelids and Molluscs, any more than 

 that the Nauplius represents the ancestor of the 

 Crustacea. 



The Echinoderms or Star-fishes, Sea Urchins, etc., 

 which were mentioned in the first chapter as being 

 among the few animals possessing a radial as opposed 

 to a bilateral symmetry, pass through a free-swimming 

 larval phase known as the Auricularia (Fig. 16), which 

 differs entirely from the Trochosphere. The Auricu- 

 laria like the Trochosphere is a small transparent 

 marine organism, with a curved alimentary canal and 

 a ciliated band, but this ciliated band is post-oral in 

 position, encircling the body with a sinuous girdle 

 which lies between the mouth and anus. Internally 

 the Auricularia is also distinguished by the possession 



