1^18] Esscndcrg: Distribuiion of the Polynoidae 227 



iioid. The color producing enzyme may be obtained by the polynoid 

 either by partly feeding on its host or the enzyme may be given off 

 the moUiisk in some kind of secretion. There may be still another 

 possibility, viz., that both the commensal and the mollusk are obtain- 

 ing the same kind of food and that this food may act as stimulant in 

 producing the same colors. Yet food alone could not be responsible 

 for the color production ; it may be only one of the agencies concerned 

 in color production, else the pigment would be distributed more or 

 less equally over the entire body. Cases are known, however, as in 

 Polynoe scolopendrina (Mcintosh, 1900) in which dark brown pig- 

 ment is produced only around the mouth and the anterior ventral 

 region, giving that part of the body the general appearance of Lysidice 

 whose tube the polynoid occupies. The rest of the body is, however, 

 less affected by the color. Be it the similar food conditions, or the 

 influence of some enzymes that produce the same combination of colors 

 in the holothurian and in the polynoid, and another color pattern in 

 the mollusk and in the polynoid, it is evident that the same forces 

 which act on the messmates in producing a certain color pattern do 

 act on the commensal polynoids producing the same results. The 

 significance here lies in the fact that animals widely different in the 

 scale of evolution possess the same properties which when aroused by 

 similar stimuli respond in the same way producing similar results in 

 pigmentation. 



Other external changes in commensals may be caused by the various 

 environmental influences in response to the plasticity of the animal. 

 The close contact with the messmates, probably through the secre- 

 tions and enzymes of the latter, may react variously on the commensal, 

 weakening the external structures such as the cuticle and elytra, so 

 that they gradually degenerate (pi. 8, figs. 27, 31) or assume a more 

 delicate texture like that of the messmate. The abundant food supply 

 which the commensal obtains and the lack of exercise aid in increas- 

 ing the bulk of the body, while the limited space in a tube aids in 

 shaping the body, thus entirely changing the external appearance 

 of the commensal. This suggests the possibility that a species, if 

 subjected to similar environmental conditions for generations may 

 change its characteristics and become a new variety or a new species. 

 In our well known Hcdosydna insignis, the free-living and the com- 

 mensal may become two distinct varieties, if not distinct species, if kept 

 in their corresponding environments for generations. These observa- 

 tions suggest the possibility that the environment and a corresponding 



