ASSOCIATED WORMS.—A considerable number of worms live in association with other animals, either as commensals or as parasites, and it is not in every case possible to decide in what relation the two animals stand. _Labrorostratus parasiticus_, a Eunicid, is parasitic in the body-cavity of _Odontosyllis ctenostomatus_ (Fig. 158); such an association between two members of the same group of animals is peculiar; but still more exceptional is the occurrence of _Haematocleptes terebellides_, as a parasite in _Marphysa sanguinea_, for both parasite and host are members of the same family, the Eunicidae. Another Eunicid, _Oligognathus bonelliae_, occurs in the body-cavity of the Gephyrean _Bonellia_. The Polynoid _Acholoe astericola_ and the Hesionid _Ophiodromus flexuosus_ occur as ectoparasites (or perhaps commensals) in the ambulacral grooves of the starfish _Astropecten aurantiacus_. An Amphinomid is stated to live in the branchial chamber of {298}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_) _bairdi_ lives between the mantle and foot of the mollusc _Fissurella cratitia_; _P. pentactes_ is found on the body of the Holothurian _Cucumaria pentactes_, and appears to be protectively coloured. _P._ (_Antinoë_) _parasitica_ lives under the elytra of another Polynoid, and _P. acanellae_ on the coral _Acanella normani_.[357] [Illustration: FIG. 158.—_Odontosyllis ctenostomatus_, with (_L_) _Labrorostratus parasiticus_ in its body-cavity. The parapodia and cirri are omitted from the greater part of the body. (After St. Joseph.) × 4.]