230 University of California PuMicaUons in Zoology [Vol. is 



9. The greatest numbers of polynoids are found in the littoral zones 

 where food is more abundant. Hence the food conditions of a certain 

 area may be of some importance in controlling distribution. The mode 

 of life may also influence the distribution. This would be especially 

 true of commensal polynoids. Their distribution would naturally 

 depend on that of their messmates. 



10. Modifications and changes in the polynoids are considerable. 

 These changes are marked in size, shape and color of the body, in the 

 sizC; shape, structure and color of the elytra, in the number, shape, 

 size and structure of the setae, and in the texture of the cuticle. These 

 changes are remarkable in commensal polynoids, so that commensal 

 individuals differ greatly from the free-living forms of the same species 

 in all the characteristics enumerated above. The deep water species 

 differ considerably from the littoral species and certain characteristics, 

 such as the thin cuticle, the smaller size, the delicate elytra and eye- 

 less condition of many abyssal species, point to the conclusion that the 

 latter have undergone a degeneration and a physical specialization 

 which makes them fit for that particular environment only. Such 

 changes, however, would be impossible if the polynoids did not possess 

 a plasticity and were not able to react in response to the external 

 stimuli. The plasticity may be therefore, regarded as one of the most 

 important factors in controlling distribution. 



11. The perfect color mimicry of the commensal polynoids sug- 

 gests that there is some chemical or enzymic interaction between the 

 messmates and the commensal, producing similar color patterns in 

 animals widely different in kind. 



12. The great uniformity of littoral species and the great diversity 

 of abyssal genera and species lead to the conclusion that the littoral 

 zone is the center of origin, or the center of dispersal of the polynoids, 

 and that species migrate or are driven to abyssal areas where they 

 undergo great specialization and partial degeneration. 



