216 University of California Puhlicotions in Zoology [Vol. 18 



the surface layers without penetrating even to a depth of 100 meters. 

 There is an absolute darkness in the depths below 1,000 meters. Add- 

 ing these combined factors to the low temperature, insufficient food 

 supply, the small quantity of free oxygen, and carbon dioxide are 

 sufficient causes to bring about variations and changes in abyssal 

 Polynoidae. 



The effect of the environment on the Polynoidae and the great 

 plasticity of the latter is especially noted in commensal polynoids. 

 The variation in size between the commensals and free-living forms 

 of the same species is remarkable in some cases. Halosydna insignis 

 is a beautiful example of this variation. This species is known as a 

 free-living form occurring in a variety of environments ; it also 

 lives as a commensal in the tubes of AmpJiitrite and Thelcpus. The 

 species is widely distributed, occurring along the entire coast of 

 North America. The commensal individuals are longer and more 

 rounded. The elytra are thinner and, excepting the first pair, devoid 

 of marginal cilia. They are smaller and do not cover the entire 

 dorsum. The spinous tubercules on the elytra are very much reduced 

 and are almost microscopic in size (pi. S, fig. 27). The dorsalmost 

 setae are greatly enlarged and bear an enlarged spur. The neural 

 setae are stouter and strongly hooked. The free-living specimens are 

 broader and shorter and usually smaller in size than the commensals. 

 The elytra are thicker, tougher and larger, strongly overlapping and 

 covering the entire dorsal surface (pi. 8, figs. 28-30). They are 

 thickly covered with large, horny prickles and bulbs. The marginal 

 cilia are longer than they are in commensals and are present on all 

 elytra ; there is also a tuft of long cilia arising a short distance from 

 the anterior margin. The setae are more slender, less strongly hooked, 

 and with fewer serrations. The pigmentation is stronger in com- 

 mensals than in free-living examples. But the pigmentation varies 

 considerably in both kinds. From all the characteristic differences 

 indicated the two forms are very likely to be taken for different 

 species rather than for members of the same species; however, these 

 characteristic differences in the commensal and free-living Halosydna 

 insignis illustrate the great plasticity in that species of polynoids and 

 the influence of the environment upon it. Thrusting itself into a 

 tube of a messmate the polynoid does not naturally find the tube made 

 to fit the shape of its body, and if it were not for the great plasticity 

 of the creature it would perish in the tube if it did not soon leave it. 

 But here comes nature to assist the intruder in adapting itself to its 



