218 Mr Cooke, On Parasitic Mollusca. [May 18, 



habitat, the mussel no doubt having become involved in the 

 branchiae of the crab in the larval form. 



On Mollusca. A species of Odostomia (pallida Mont.) is found 

 on our own coasts on the 'ears' of Pecten maximus, and also 1 on the 

 operculum of Turritella communis. At Panama the present writer 

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

 Strombus galea and of Cerithium irroratum. Amalthea is very 

 commonly found in Conus, Turbo, and other large-sized shells, but 

 this is probably not a case of parasitism, but simply of con- 

 venience of habitat, just as young oysters are frequently seen on 

 the carapace and even on the legs of large crabs. 



On Tunicata. Lamellaria is said to deposit its eggs on an 

 Ascidian (Leptoclinum), and the common Modiolaria marmorata 

 lives in colonies imbedded in the tegument of Ascidia mentula 

 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 in the 

 nutritive material. A colourless shell is not necessarily pro- 

 tective, for though a transparent shell might evade detection, 

 a milk-white hue would probably be conspicuous. (2) Modifica- 

 tions of structure. These are in many cases considerable. Ento- 

 concha and Entocolax have no respiratory or circulatory organs 

 and no nervous system ; Thyca and certain Stiliferi possess a 

 curious suctorial apparatus ; the foot in many cases has aborted, 

 since the necessity for locomotion is reduced to a minimum 2 , and 

 its place is supplied by an enormous development of the proboscis, 

 which enables the creature to provide itself with nutriment with- 

 out shifting its position. Special provision for holding on is 

 noticed in certain cases, reminding us of similar provision in 

 human parasites. Eyes are frequently, but not always wanting, 

 even in endo-parasitic forms. A specially interesting modification 

 of structure occurs in ,(3) the Radida. In most cases (Eidima, 

 Stilifer, Odostomia, Entoconcha, Entocolax, Magilus, Coralliophila, 

 Leptoconcha) it is absent altogether. In Ovula and Pedicidaria, 

 genera which are in all other respects closely allied to Cypraea, 

 the radula exhibits marked differences from the typical radula of 

 the Cypraeidae. The formula (3'1\3) remains the same, but the 

 laterals are greatly produced and become fimbriated, sometimes 

 at the extremity only, sometimes along the whole length. A 



1 Smart, Journal of Conch, v. 152. 



2 Semper notices a case where a Eulima whose habitat is the stomach of a Holo- 

 thurian retains the foot unmodified, while a species occurring on the outer skin, but 

 provided with a long proboscis, has lost its foot altogether. 



