THE GASTROPODA 73 
/forms:such as the Nudibranchs, the Cymbuliidae, and Plewrobranchus, 
are provided with an operculated shell during their development, 
the only exceptions being some highly specialised forms, ¢.g. the 
Pulmonates (excepting the Auriculidae, Siphonariidae, and On- 
cidiidae, which have an operculum during development), Luncina, 
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, Voluta, 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. helicina. 'The composition 
of the operculum varies very 
much in the different groups 
of Gastropoda. It is com- 
Fic. 49. 
Limacina antarctica, young Fic. 50. 
specimen with the operculum : 
in situ. f.sp, false spire; op, Cyclostrema decussatwm. Shell and oper- 
operculum, culum (0p). 
monly horny, or it may consist of a horny plate covered by a thin 
calcareous layer, as in Liotia among the Delphinulidae and Cistulu 
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 may be 
furnished with lateral apophyses as in Neritina, Rissoina, 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 Hipponyax 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 
