298 



POLYCHAETA 



CHAP. 



the barnacle, Lepas anatifertu Alciopina parasitica lives, during 

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



that most of the Alciopids thus make 

 of Ctenophores as their nursei 



A considerable number of the Polynoids 

 are ectoparasitic : P. castanea lodges in tl 

 peri-oral region of Spatanyus purpur 

 in the ambulacral grooves of Astropetten ; 

 (Halosydna) bairdi lives between the mantlt 

 and foot of the mollusc FissureUa cratitia ; P. 

 pentactes is found on the body of the H !< 

 thurian Cucumaria pcntactes, and appears 

 be protectively coloured. P. (Antinoe) 

 sitiea lives under the elytra of anoti 

 Polynoid, and P. acaneNae on the coi 

 ella norma hi. 1 

 As commensals there may be mention 

 >eaia y which lives in the upper 

 of whelk-shells which are inhabited by 

 hermit crab. The same shell usually 

 a particular sea -anemone, so that there 

 three animals living together in or upon 

 ^ cast-off house of a fourth. Siphonostoma 

 found in the "nests" made by the niolli 



Fig. 158. 



w 



in its 

 bodr-caTity. 

 parapodia and cirri 

 are omitted from the , 

 greater part of the ^*wi#. A It unite is constantly associate 



body. (After St. with the coral Lophohflia prolifera, anioi 



Joseph.) x-L , , _ _ \ . . ^ 



the branches of which the worm twines it 

 tube : whilst another Polychaete inhabits a tube formed by tht 

 interweaving of the fine branches of the coral Antipathes flix? 

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

 tube in Heliopora. The Polynoids present many instances of eoni- 

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

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

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

 in tubes of the Eunicid Jfarphysa sanyuinea. Two species 

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

 iu tubes of Srrpula rermicalaris, while P. artnieolae occurs 



1 For a list of parasitic Polyehaetes see St. Joseph, An*. Sei. JYof. (ser. 7) v. 1888, 

 p. 141. 



2 Semper, loe. n't. p. 340. 



