^918] Esscnherg: Distrihution of the Poljjnoidae 213 



paratively few animals can exist on a given area, while an area 

 similar in size but with abundant food supply nearer shore naturally 

 contains an abundance of animals. 



Aside from the problem of food supply the mode of life of the 

 Polynoidae deserves some consideration. The majority of the Poly- 

 noidae are free living forms and the largest numbers, as has been 

 indicated above, are found near the shores and between tide marks 

 where they hide in crevices and beneath rocks, pebbles and weeds or 

 crawl about freely. Most probably they do not go very far from 

 their dwelling place unless driven away by some physical force. 



There are, however, some polynoids which depend for their dis- 

 tribution on the locomotion and the mode of life of other animals. 

 These are the commensal forms. The number of strictly commensal 

 species known up to present time is limited. Nevertheless, some 

 species are known exclusively as commensals and depend for their 

 distribution on their messmates; hence their distribution naturally 

 coincides with that of the latter. These commensal polynoids are : 

 Lepidasthenia gigas, commensal with a large tube dwelling annelid, 

 Aniphitrite; Pohjnoe lordi, commensal with the limpet, Glyphis aspera 

 and with Cryptochiton stelleri, in which it occupies the branchial 

 groove ; Polynoe fragilis, commensal with the starfish, Asterms 

 ochracea. These species are known up to the present time only as 

 commensals, and have a very limited distribution. A few other species 

 occur both as free living forms and occasionally as commensals. Those 

 known on this coast are : Halosydna insignis, Polynoe calif ornica and 

 Harmothoe imhricata. These species do not necessarily depend on 

 their messmates since they live both as commensals and independently. 

 They are most widely distributed on this coast. Evidently the ability 

 to live in a variety of environments favors the wide distribution of 

 these polynoids. The known facts and the observations lead one to 

 the conclusion that the food habits, food supply and the mode of life 

 play an important role in determining the distribution of the Poly- 

 noidae. 



VII. Plasticity 



Some of the most obvious external factors controlling distribution 

 have been enumerated. But there may be other factors of a more 

 complex nature, w^hich are beyond our present reach and are waiting 

 for future investigations. There is, however, one more factor, viz., 

 variability. Variability or plasticity may be considered both as a 



