138 THE BIOLOGY OF THE SEA-SHORE 



having much of its catch abstracted by the crab (Duerden, 



1905)- 



Although we have purposely refrained hitherto from 



introducing the Protozoa we cannot leave the subject of 



commensalism without a reference to the many forms of 



single-celled animals (Infusoria) which occur in or on the 



bodies of a number of shore forms, and, since there is no 



evidence of their producing injurious eifects, are usually 



spoken of as being commensal with the animals they inhabit. 



Fig. 1 1 . — a, Crab {Melia tessellata) bearing in its claws two anemones ; 

 b, claw much enlarged. (From The Fauna and Geography of the Maldive 

 and Laccadive Archipelagoes, after Borradaile.) 



They are particularly numerous, for instance, in the gut of 

 Polychaet worms, and on the gills of molluscs. Issel (19 18) 

 mentions various species of Protozoa commensal with 

 Littorina. According to this writer, the commonest forms 

 are of oval shape, flattened and very active. They move 

 about over the surface of the gill lamellae. The individuals 

 of a less common species are usually united in small groups, 

 and are fixed to the gills of the host by the base of their 

 cylindrical body ; their free extremity is spherical and sur- 

 rounded by a ring of cilia. Issel names this second species 

 Scyphidia littorince. 



