268 



THE OPERCULUM 



and Patella have reached the stage at which the operculum has 

 been lost entirely. In Navicella^ besides becoming degraded, the 

 operculum has actually become partly internal, and apparently 

 serves the purpose of separating the viscera from the upper 

 part of the foot, something like the shelly plate in Crepidula. 

 This explains why the operculum in this genus is polished on 

 both sides.^ 



Some authors have imagined that the operculum is homo- 

 logous (a) to the second valve in Pelecypoda, (5) to the byssus. 

 It differs, however, morphologically from the former in the 

 essential point of not being produced by the mantle, and from 

 the latter in not being produced by a special gland. 



As regards shape and formation, the operculum has usually 

 a more or less well-marked nucleus which may be central (e.g. 

 Livona')^ subcentral (^Ampullaria^^ lateral (^Purpura'), or terminal 



Turbo 

 (Sarmaticus) 



Livona 



Turbo 

 (Callopoma) 

 Fig. 182. — Various forms of opercula. 



pdllaria 



Natica 



(^Pyrula^. As a rule, -both the inner and outer surfaces are fairly 

 fiat, but in Torinia^ Cyathopoma^ and Pterocyclus the outer sur- 

 face is elevated and conically spiral, in some Turho (e.g. Sarmati- 

 cus') it is covered with raised tubercles resembling coral, while in 

 others (e.g. Callopoma) it is scored with a deep trench. Aido- 

 poma^ a land genus peculiar to Ceylon, has a paucispiral oper- 

 culum with hollow whorls, deceptively like a Planorhis ; it fits 

 over the aperture instead of into it. In Livona and most 

 Trochidae the operculum is cartilaginous and multispiral. In 

 Stromhus it is narrow, curved, and often serrated like a leaf 

 on one of the edges ; in Conns it is narrowly oblong and rather 

 featureless ; in Littorina^ paucispiral and always cartilaginous. 

 1 J. E. Gray, Phil. Trans. 1833, p. 812. 



