134 THE BIOLOGY OF THE SEA-SHORE 



of the parasitic habit and affords an instance of hyperpara 

 sitism (the parasitisation of a parasite) . This little Gasteropod 

 is found exclusively among the old and resistant byssus 

 threads of adult mussels which live affixed in cracks of the 

 rocks in the east and south of La Tour de Croy, at Wimereux, 

 etc. They are held firmly to their support by a slender 

 mucous thread secreted by a foot gland, and may be so found 

 during low-tide. This species of Odostomia has no radula 

 but is furnished with a long sucking tube. Experiment has 

 shown that whenever the snail was placed in a basin together 

 with a fresh mussel it would first remain in the byssus, and 

 then climb upon the shell of the bivalve, about the posterior 

 or ventral edge ; on arriving there the animal would extend 

 its tube and introduce it in a curved manner between the 

 valves. Endeavours to infect Tapes, TelHna, and Mactra 

 gave negative results. The Odostomia reaches 4 to 5 mm. 

 in size. It is itself parasitised by a Copepod {Momtrilla 

 helgolandica), which occupies the anterior region of the body. 

 In parenthesis, it may be noted that O. rissoides, living as 

 it does with the mussels above the limit of low tide, achieves 

 its complete embryonic development within the egg capsule 

 and does not possess a free pelagic stage, in the form of a 

 veliger larva, like its relatives (Pelseneer, 19 14). 



We have already commented on the number of different 

 organisms to be found on the surface of some of the larger 

 Crustacea (see p. 90). As a further example of the hetero- 

 geneous collection of forms to be found associated with a 

 single species we may take the case mentioned by Pearse 

 (1913), where a single crab {Carcinus horealis) bore on the 

 dorsal surface of the carapace the tube-worm Spirorhis and 

 the alga Hildenhrandtia ; on the right side anterior to the 

 legs, a specimen of the limpet Crepidida plana and its eggs ; 

 on the legs, the tube-worm Spirorhis and a small Crepidula, 

 and under the abdomen three mussels {Mytilus). 



The common hermit crab Eupagiirus hernhardus, the 

 younger forms (seldom the adults) of which are so abundant 

 between tide-marks, makes use of shells of forms such as 

 Littorina, Trochus, and Purpura which themselves live 



