182 University of California Puhlications in Zoology [Vol. is 



1. Littoral Species 



It is a general phenomenon that oceanic animals, as far as the num- 

 ber of individuals is concerned, are more abundant in littoral zones 

 than in deep water. This is true of Polynoidae, some littoral species 

 of which are very numerous. This, however, is not true of the number 

 of genera and species, greater numbers of groups being represented 

 in deep waters as is shown in table 4, where fifteen species are given 

 as exclusively littoral. The fifteen littoral species belong to four 

 genera. There are, moreover, fourteen exclusively deep water species 

 and seven genera, the species being equal and the genera being almost 

 double the number of those of the littoral zones. The explanation of 

 this may be that the littoral zone being the location of greatest abun- 

 dance of individuals is the center of origin of the Polynoidae and 

 their center of dispersal. 



That the littoral zone is the center of dispersal may be assumed on 

 the basis that the individuals are very numerous in that zone and that 

 the species are less varied or more nearly related. On the other 

 hand the great diversity of species in the deep waters may be attri- 

 buted to the environmental change and to the sudden shock efi^ect on 

 the larval stages of the Polynoidae. These changes may be brought 

 about in larvae that are driven from the shores to the deep waters 

 undergoing there considerable change in temperature, especially when 

 they drop to the bottom ; the shock eif ect of the cold temperature most 

 probably kills the majority of the larvae and the few survivors that 

 are able to adapt themselves are physically so changed as to form 

 new species or even new genera. 



Or again, if adult polynoids are driven to depths, they possessing 

 a greater power of resistance than the larvae, have more chance to 

 survive but their offspring will undergo a considerable change. In 

 the first place, the low temperature at the bottom of the ocean would 

 naturally cause death and abnormalities among the embryonal poly- 

 noids and when rising to the surface as trochophores they are sub- 

 ject to a sudden temperature change. The percentage of mortality in 

 this case would be very great, and abnormalities resulting in. con- 

 spicuous variations would be great. This assumption is based on 

 laboratory experiments in which similar results may be produced arti- 

 ficially by suddenly changing the temperature. 



The littoral species most common on this coast occurring in less 

 than 30 fathoms depth are not strictly confined to that bathymetric 



