224 University of California Publications in Zoology [Vol. is 



The greatest variation and plasticity is seen in the commensal 

 pol.ynoids. As a general rule the commensals are larger in size than 

 the free-living individuals of the same species. This fact is un- 

 doubtedly due to the greater food supply which a connnensal obtains 

 by robbing its messmate of the food which the latter has secured, 

 or the commensal may become an ectoparasite as has been observed 

 to be the case with some polynoids living on sponges. In examining 

 the contents of the stomachs of polynoids (Darboux, 1899) spicules 

 of sponges were found indicating that the commensals had been feed- 

 ing on their messmates. Secondly, a commensal is protected by the 

 messmate by living in the tube or hiding in some sheltered place of 

 the body of the latter. Since it lives there in absolute quiescence 

 there is less catabolism in its organism than in the free-living worm 

 which has to exert a great deal of its energy in securing food and in 

 watching for its own safety. Consequently almost the wliole food 

 supply of the commensal goes to build up and to increase the bulk 

 of the body, and the natural result is the larger size of body which has 

 been mentioned above. 



Moreover, the shape of the body of the commensals may be greatly 

 modified by an increase in length, and by reduction in breadth, thus 

 changing from a short, compressed to a long, round form. The pro- 

 tective structures in commensals, as the elytra, the cuticle and the 

 setae, are usually degenerate. 



A marked adaptive variation is noticeable in the number, struc- 

 ture and size of the setae according to the mode of life of the com- 

 mensal. In the free-living species the setae are numerous, usually 

 from 50 to 100 on each parapodium (pi. 7, figs. 1, 19). The setae 

 in the free-living species are also rougher, with numerous, strong 

 serrations (pi. 7, figs. 4-6, 8-11). In commensals and ectoparasites, 

 especially in those which are in the habit of attaching themselves to 

 their hosts, the number of setae is greatly reduced (pi. 7, figs. 15, 

 17). Some species, as Polynoe pulchra and Polyvoe lordi, have only 

 5 or 6 slender, -smooth, sharply pointed setae, the blunted setae being 

 constantly replaced by new, sharply pointed ones which arise from 

 the base of the parapodium (pi. 7, figs. 3, 7). In some of the tube- 

 dwelling commensals, as Lcpidasthcnia gigas, the number of the setae 

 is small, usually 5 to 8 on each parapodium. The setae (pi. 7, fig. 13) 

 have a few serrations and are not so sharply pointed as are those in 

 the commensals which use their setae for attachment to their host. 

 In Lepidasthenia gigas, hoAvever, there is, in addition to the ordinary- 



