l^lS] Essenherg: Distnbulion of the Polynoidae 229 



F. SUMMARY 



1. Polynoidae as a group have a world wide distribution, occurring 

 in all oceanic waters. The fifty-one species of Polynoidae on this 

 coast are divisible into two main divisions: (1) the cosmopolitan, and 

 (2) the non-cosmopolitan species. The cosmopolitan species occur 

 in all oceans ; the non-cosmopolitan are restricted to the Pacific Ocean 

 and many of them occur only along the shores of North America. 



2. The Pacific coast polynoids, on the basis of their geographical 

 distribution, fall into boreal, north temperate, and subtropical species, 

 according to the life zones they occupy. 



3. The polynoids are again divisible into littoral, sublittoral, and 

 abyssal species according to their bathymetrieal distribution. 



4. The distribution of Polynoidae is evidently controlled by certain 

 factors. The most important factors determining distribution are : 

 {a) temperature, (&) currents, (c) winds, {d) chemical composition 

 of water, {c) food habits and mode of life, and (/) plasticity or 

 response of the animal to its environment. 



5. The facts that certain species are restricted to definite life zones 

 and to definite ranges of temperatures, that the same species occurring 

 in widely separated areas occur in depths of similar temperature and 

 that boreal species occur in temperate and subtropical zones as deep 

 water species, seem to point to the conclusion that temperature is the 

 chief factor in determining distribution. 



6. Some species of Polynoidae are found in widely separated areas. 

 Since they cannot possibly traverse such distances by their natural 

 means of locomotion, currents may be regarded as agents in distribu- 

 tion, carrying the adult worms along the bottom and the pelagic 

 larvae in the upper strata and at the surface. The currents may 

 further influence the temperature and the food conditions of an area. 

 Consequently they play an important role in determining distribution. 



7. After severe storms some deep water species are usually found 

 driven to the shores. The winds may thus serve as agents in determin- 

 ing distribution. 



8. The chemical composition of waters differs at diiferent depths 

 and latitudes. Since- certain species of polynoids are restricted to 

 definite areas and to definite depths, undergoing there considerable 

 external changes, the chemical composition of water may be of some 

 importance in determining distribution. 



