178 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



and the great key-hole limpet, Lucafina crenulata. In 

 case of Lucapina crenulata, Polynoe -pulchra nestles in the 

 cavity between the mantle-flap and the foot; but in case of 

 the holothurian, it clings to the surface of its host, and even 

 at times crawls into its mouth. The hooked ventral seta 

 serve admirably to enable the animal to maintain its hold. 

 In fact, it clings so closely one has to exercise care in re- 

 moving a specimen or it will be ruptured. The animal will 

 live a long time — how long I have not ascertained, but at 

 least two weeks — after separation from its host, if the water 

 be kept pure. 



The astonishing variation of color which this species 

 exhibits is in exact agreement with its varied mode of life. 

 Under the mantle of Lucafina it is securely hidden 

 from the observation of any inquisitive fish, and almost 

 excluded from the light. In this situation it may either be 

 destitute of pigment, or possess a color-pattern that would 

 render it exceedingly conspicuous in almost any external 

 environment. Nothing could be more chastely beautiful 

 than the translucent cream-color, accentuated in the purity 

 of its tint by the rich velvety brown of the dorsum and the 

 jet-black of the elytra; and nothing could be more fatal to 

 the creature if it were exposed to the attacks of predatory 

 animals. It is not surprising, therefore, to find a differ- 

 ent coloration in case of individuals that lead an exposed 

 existence on the bodies of holothurians. In this instance 

 the color of the parasite mimics that of its host. It is fur- 

 ther interesting to note how slight a change is necessary 

 in the pattern-coloration to render the animal inconspicuous. 

 It is merely a washing over of the surface with reddish or 

 brownish — the color of the holothurian. Then the spots 

 are toned down to a lighter shade of brown, or from 

 intense black to dark brown. The result is a very good 

 color-mimicry of the holothurian. 



The structure of the seta? in this species, in P. lordi and 

 P. fragilis, is a beautiful adaptation to their ectoparasitic 

 habit. The serrations are few and very diminutive ; the 

 tip is decidedly hooked, the number of seta? is reduced, 



