. The biology of marine animals. Marine animals; Physiology, Comparative. 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. ). Somewhat similar is the relation between a peculiar hydroid Proboscidactyla and various sabellid worms (Fig. ). The hydroid attaches itself about the mouth of the worm's tube,
. The biology of marine animals. Marine animals; Physiology, Comparative. 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. ). Somewhat similar is the relation between a peculiar hydroid Proboscidactyla and various sabellid worms (Fig. ). 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. 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 The worm usually lies withdrawn in the upper coils of the shell, where it maintains a current of water by steady or intermittent pulsations of its body.
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