78 



MOLLUSCA PARASITIC ON MOLLUSCA 



CHAP. 



Fig. so.— Crepidu- 

 la onyx Sowb., 

 parasitic on the 

 operculum of 

 Stromhus gale- 

 atus Swains., 

 Panama. 



under the carapace of the common shore-crab (Carcinus mae- 

 nas^, and one case has been noticed ^ where two mussels, one of 

 several months' growth, the other smaller, well 

 secured by their byssi, were found under the 

 abdomen of the same species, in such a position 

 as to force the appendages apart and askew. 

 These, however, are not so much cases of para- 

 sitism as of involuntary habitat, the mussel no 

 doubt having become involved in the branchiae 

 and the abdomen of the crab in the larval form. 

 On MoUusca. — A species of Odostomia (^pal- 

 lida Mont.) is found on our own coasts on the 

 'ears' of Peeten maximus, and also^ on the oper- 

 culum of Turritella communis. Another species 

 (0. rissoides) frequently occurs in hiding under 

 beds of mussels, but it is not clear whether the 

 habitat is due to parasitism, or simply to the 

 fact that the mass of mussels, knitted together 

 and to the rock by the byssi, affords the Odosto- 

 mia a safe lurking-place. At Panama the present writer found 

 Crepidula (2 sp.) plentiful on the opercula of the great Strom- 

 bus galea and of Cerithium irroratum,. In each case the parasite 

 exactly fitted the size of the operculum, and had assumed its 

 colour, dark brown or chestnut. Amalthea is very commonly 

 found on Conus, Turbo, and other large shells from the South 

 Pacific, but this is probably not a case of parasitism, but simply 

 of convenience of habitat, just as young oysters are frequently 

 seen on the carapace and even on the legs of large crabs. 



On Tunicata. — Lamellaria deposits its eggs and lives on an 

 Ascidian (^Leptoclifium'), and the common Modiolaria marmorata 

 lives in colonies imbedded in the test of Ascidia mentula and 

 other simple Ascidians. 



Special points of interest with regard to parasitic MoUusca 

 relate to (1) Colour. This is in most cases absent, the shell 

 being of a uniform hyaline or milky white. This may be due, 

 in the case of the endo-parasitic forms, to absence of light, and 

 possibly, in those living outside their host, to some deficiency in 

 the nutritive material. A colourless shell is not necessarily pro- 



^ W. Anderson Smith, Loch Creran, p. 46. 

 2 Smart, Journal of Conch, v. p. 152. 



