THE GASTROPODA 



73 



forms such as the Nudibranchs, the Cymbuliidae, and Pleurobranchus, 

 are provided with an operculated shell during their development, 

 the only exceptions being some highly specialised forms, e.g. the 

 Pulmonates (excepting the Auriculidae, Siphonariidae, and On- 

 cidiidae, which have an operculum during development), Buncina, 

 Cenia, and of the " Pteropods," the Cavoliniidae and Gymnosomata. 

 An operculum may be present or absent in the adults of the 

 same genus, as may be seen in Stomatella, Vermetus, Valuta, Mitra, 

 Pleurotoma, and Conus. It may be absent in certain individuals of 

 the same species as in Volutharpa ampullacea, or it may be normally 

 caducous in aged individuals 

 as in Limacina antarctica and 

 L. kelicina. The composition 

 of the operculum varies very 

 much in the different groups 

 of Gastropoda. It is com- j^^^^^^&m ,Ofl 



\ 



Ofl 



Fio. 49. 



Lirnacina antarctica, young 

 specimen with the operculum 

 in situ, f.sp, false spire ; op, 

 operculum. 



FIG. 50. 



Cyclostrcma decussatum. 

 culuui (op). 



Shell and oper- 



monly horny, or it may consist of a horny plate covered by a thin 

 calcareous layer, as in Liotia among the Delphinulidae and Cistula 

 among the Cyclostomatidae ; or, finally, it may be completely calcified, 

 as in the Turbinidae, Phasianellidae, Neritidae, etc. Its conformation 

 is originally spiral, and in this case the spire is always inverse to 

 that of the shell, even in the Atlantidae (Fig. 48), except in certain 

 cases of hyperstrophy described below. It may, however, be con- 

 centric, imbricated, or scaly (Strombidae, Fig. 75, op), and it maybe 

 furnished with lateral apophyses as in Neritina, Eissoina, and Stiva. 

 Some non-operculate testaceous Gastropods, as, for instance, many 

 stylommatophorous Pulmonates and some species of Planorbis, 

 secrete glutinous or calcareous epiphragm which closes the mouth 

 of the shell during hibernation or aestivation. In Hipponyx the 

 foot secretes a calcareous plate by means of which the animal 

 fixes itself to the substratum. 



2. Visceral Sac, Mantle, and Shell. The mantle normally covers 

 the whole of the visceral sac and projects all round it, leaving only 

 the head and foot projecting on the ventral side. On the anterior 

 or on the lateral aspect, or exceptionally on the posterior aspect, as 



