ASSOCIATIONS 



589 



ambulacral groove of the burrowing starfish Astropecten irregularis. The 

 polynoid Harmothoe adventor, which lives in the burrow of the echiuroid 

 worm Urechis caupo, derives not only protection from the relationship but 

 food as well, since it seizes some of the mucus-bag by which its host carries 

 out filter feeding (Fig. 14.6). Somewhat similar is the relation between a 

 peculiar hydroid Proboscidactyla and various sabellid worms (Fig. 14.7). 

 The hydroid attaches itself about the mouth of the worm's tube, pilfers 

 some of the food gathered by the ciliary activity of its host and occasion- 

 ally even seizes some of the latter's eggs (17a, 25, 41, 74). 



Experimental analyses of host-commensal relationships in polynoid 

 worms have shown that some species are attracted by specific substances 

 given]offby their[co-partners. Thus Arctonoe fragilis, which is a commensal 



Fig. 



14.7. Proboscidactyla stellata (= Lar sabellarum), a Commensal Hydroid 

 Living on the Tube of a Sabellid Branchiomma vesiculosum 

 (After Gosse, 1877.) 



of various starfishes Evasterias, etc., is attracted by the scent of its host 

 and can distinguish between water coming from its host and sea water 

 alone. Moreover, the attraction is a specific one, and Arctonoe from one 

 host species is not attracted to other species of starfish. Other com- 

 mensal polynoids give positive responses to host and related species. 

 Polynoe scolopendrina, for example, is attracted to its normal host, Polymnia 

 nebulosa, and to other terebellids in lesser degree. The chemical attractants 

 appear to be unstable or closely bound substances in the partnerships 

 investigated (18, 43«). 



An often cited commensal is Nereis fucata which lives in whelk shells 

 inhabited by hermit crabs (Eupagurus bernhardus). The worm usually lies 

 withdrawn in the upper coils of the shell, where it maintains a current of 

 water by steady or intermittent pulsations of its body. When the hermit 

 crab is manipulating a piece of food, however, the worm extends its head 

 outside the shell, and seizes some of the food from the crab's mandibles 

 (10, 24). Gosse (36) has described the activities of the worm as follows. 



While I was feeding one of my Soldiers by giving him a fragment of cooked meat, 

 which he having seized with one claw had transformed to the foot-jaws, and was 

 munching, I saw protrude from between the body of the Crab and the Whelk-shell 



