1918] Essenhcrg: Distvihution of the Pohpioidae 225 



setae, a strongly developed dorsal neuro-seta (pi. 7, iig. 18) peculiar 

 to that species only. Whether it has any significance in the coni- 

 mensalistic life of that polynoid is an open question. Whether the 

 reduction of setae is a secondary characteristic developed in response 

 to commensalistic life, or whether the polynoids chose that mode of 

 life because of the fitness of their setae for that purpose, is another 

 question which cannot be definitely settled at present. However, the 

 sharply pointed and hooked setae are of special use to the polynoid 

 as a means of attachment to the host. 



One phase of plasticity in polynoids reveals itself in the variation 

 of color in response to different environments. In the majority of 

 cases the abyssal forms are less brightly colored. The commensals 

 and ectoparasites, hoM'ever, display a great variation in color, as is 

 seen in some species which occur simultaneously on two or more differ- 

 ent hosts. For example, Polynoe pulchra, which lives on Lucapina 

 crenulata hidden between the foot and the mantle flap of the latter, 

 is very conspicuously colored, while individuals of the same species 

 found on holothurians, completely mimic the color of the latter. 



Such changes in color may be due to various causes but in each 

 case are evidently caused in response to some chemical or physical 

 stimulus. The color in each animal indicates that there is some 

 chemical reaction particular to that species. With changes in the 

 environment the physical equilibrium is disturbed and some chemical 

 affinities may be more stimulated to reaction than others. These 

 changes may induce new interactions in the tissue and protoplasm of 

 the animal, resulting in external changes, such as changes in color. 

 Polynoidae in great depths are, as a rule, less strongly pigmented or 

 entirely unpigmented. This fact shows that there is some inhibitory 

 action which prevents the pigment from forming. Whether it is the 

 absence of light, the different chemical composition of the water, or 

 the low temperature that produces the changes in color cannot be 

 definitely decided. It has been proved by experimental methods that 

 no pigment was produced in the chrysomelid beetles when the embryos 

 were kept in low temperature (0° to 5°C) while embryos kept in high 

 temperature (43° to 45°C) developed dark pigment (Tower, 1906). 

 Since pigmentation involves the process of oxidation or metabolism, 

 the temperature undoubtedly is an important agent in accelerating 

 or retarding the development of color. Furthermore, the light intens- 

 ity may be of some importance in production of color. However, the 

 light reaction seems to be of secondary importance as is shown in the 



