7 8 MOLLUSCA PARASITIC ON MOLLUSCA chap. 



under the carapace of the common shore-cral,^ {Carcinus macnas), 



and one case has l)een noticed ^ where two mussels, one of several 



months growth, the other smaller, well secured 



hy theh^ byssi, were found under the abdomen 



of the same species, in such a position as to 



\ force the appendages apart and askew. These, 



j] however, are not so much cases of parasitism 



r^v '■ as of involuntary habitat, the mussel no douljt 



having become involved in the branchiae and 



the al)domen of the crab in the larval form. 



Ai On MuUusca. — A species of Odostoviia 



} {2JaUida Mont.) is found on our own coasts on 



the ' ears ' of Pecten maximus, and also - on the 



operculum of Turritclla communis. Anotlier 



Fig 30 — Creniduia species (0. rissoides) frequently occurs in hiding 



onyx Sowi)., para- under beds of mussels, but it is not clear 



sitic on the oper- i ,i ;i i -i -j. i_ • i j -i.- 



cvdum of Strombus ^vhetller the habitat is due to parasitism, or 

 fjaleatus Swuius., simply to the fact that tlie mass of mussels, 



Ptlllilllll 



knitted together and to the rock by the byssi, 

 affords the Odostomia a safe lurking-place. At l^inama the 

 present writer found Crepidula (2 sp.)^ plentiful on the opercula 

 of the great Stromhtis 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. Anialihca is 

 very commonly found on Coptics, Turbo, and otlier large shells 

 from the South Pacific, but this is prol)ably not a case of 

 parasitism, but simply of convenience of habitat, just as young 

 oysters are frequently seen on the carapace and even on tlie 

 lews of large crabs. 



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

 Ascidian {Lej^toclinuyii), and the common Modiolaria marmorata 

 lives in colonies imbedded in the test of Ascidia mcntida and 

 other simple Ascidians. 



Special points of interest with regard to parasitic Mollusca 

 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 



^ "\V. Anderson Smith, Loch Crcraii, p. 4G. 

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



