298 



POLYCHAETA 



the barnacle, Lepas anatifera. Alciopina parasitica lives, during 

 the early stages of its life-history, within Cydippe, and it is possible 

 that most of the Alciopids thus make use 

 of Ctenophores as their nurseries. 



A considerable number of the Polynoids 

 are ectoparasitic : P. castanea lodges in the 

 peri-oral region of Spatangus purpureus, and 

 in the ambulacral grooves of Astropecten ; P. 

 (Halosydna) lairdi lives between the mantle 

 and foot of the mollusc Fissurella, cratitia ; P. 

 pentactes is found on the body of the Holo- 

 thurian Cucumaria pentactes, and appears to 

 be protectively coloured. P. (Antinoe} para- 

 sitica lives under the elytra of another 

 Polynoid, and P. acanellae on the coral 

 Aca. ella normani. 1 



As commensals there may be mentioned 

 Nereis fucata, which lives in the upper coil 

 / \ of whelk-shells which are inhabited by a 



^ ^ hermit crab. The same shell usually bears 



a particular sea-anemone, so that there are 



(L) Labrorostratus three animals living together in or upon the 



parasiticus in its a> i /> j> L -\ a- 7 



body -cavity. The cast-off house of a fourth. Siphonostoma is 

 parapodia aud cirri found in the " nests " made by the mollusc 



are omitted from the ,. . . ,, ", . , 



greater part of the Lima. A J&umce is constantly associated 

 t ' w ^k ^ e cora * Zophohelia, prolifera, amongst 

 the branches of which the worm twines its 

 tube ; whilst another Polychaete inhabits a tube formed by the 

 interweaving of the fine branches of the coral Antipathes filix? 

 found in the "West Indian seas. A species of Polydora forms its 

 tube in Hdiopora. The Polynoids present many instances of com- 

 mensalism, a few of which may be here mentioned. P. johnstoni 

 Marenz. is only found in the tubes of Terebella nebulosa ; other 

 species occur in the tubes of other Terebellids. P. marphysae 

 lives in tubes of the Eunicid Marphysa sanguinea. Two species 

 live in the tubes of Chaetopterus. P. extenuata has been found 

 in tubes of Serpula vermicularis, while P. arenicolae occurs on 



1 For a list of parasitic Polychaetes see St. Joseph, Ann. Sci. Nat. (ser. 7) v. 1888, 

 p. 141. 



2 Semper, loc. cit. p. 340. 



