204 



MARINE ANIMALS 



to preserve a number of primitive forms; Chiton, and the snails Patella, 

 Fissurella, Nerita, and Haliotis, are primitive in structure and con- 

 fined to the surf zone. 



The adaptation of Littorina to unfavorable conditions extends to 

 its reproduction. L. litorea living near the low tide mark lays eggs 





■i Mytilus edulis L. |g| Haliotis tuberculata 4++ Tapes decussata L. Illl Cardium edule L 



Fig. 37.— Distribution of some index forms in the littoral of the Island of Batz 

 and the neighboring coast of Brittany. After Joubin. 



which hatch into trochophore larvae, and develop in turn into a veliger 

 stage; in L. obtusata, living between the tide marks, the development 

 is shortened; the young leave the eggs in the veliger stage; finally, in 

 L. rudis and L. neritoides, which live at high tide mark, the larval 

 stages are done away with, as these snails have become viviparous. 39 

 These progressive changes are marked by decreased fertility. 



The selective action of the surf shows itself in other ways. An iso- 



