THE GASTROPODA 135 
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Siphonariidae), the Cavoliniidae, and the Gymnosomata. The 
pedal glands are formed by ectodermic invaginations, and in some 
Species exist in the larva though they are absent in the adult 
(Purpura). 
The preconchylian invagination or shell-gland (Fig. 110, sh.gl) 
appears at the beginning of development in the centro-dorsal area 
behind the velum, on the side of the body opposite to the blastopore. 
It is surrounded by a ridge which gradually extends over the visceral 
sac and secretes the shell (Fig. 117, sh). In some Stylommatophora 
viz. Clausilia and Succinea, a pallial sac is formed which covers up 
the shell, but eventually opens again. The shell grows in thickness 
internally, fresh matter being added to it from the external surface 
of the mantle, but its increase in extent is dependent on the 
activity of the border of the 
mantle, where there are special 
glands which degenerate when the 
animal reaches the adult state. 
It is only at this period that the 
aperture of the shell acquires a lip, 
A 
Se 
JA 
VY 
Fig. 114. 
Embryo of Vermetus, ventral NSS LLG 
aspect. I, velum ; IT, contractile Larval shell of Nassa reticulata, 
sinus (“embryonic heart”); III, ventral aspect, x 30. h, hook of 
opening of the pallial cavity; IV, the dorsal edge of the aperture ; si, 
Shell ; V, foot; VI, left eye. (After future canal or shell-siphon; sp, 
Salensky.) spire. 
or 1s contracted in various ways to form, for example, the linear 
aperture of Cypraea, Cavolinia, etc. In Gastropoda that are naked 
in the adult state, the shell falls off soon after the reduction of the 
velum (Fig. 116), but in Cenia, Runcina, and Vaginula the shell- 
gland and shell are not developed, and the young animal, at the 
time of escaping from the egg, has already the naked form of the 
adult. 
The superficial contractile sinuses are portions of. the wall 
of the body, temporarily modified to ensure the circulation of the 
nutritive fluid in the system of cavities destined to become the 
circulatory apparatus of the adult. In the walls of these sinuses 
are muscular elements, whose fibres are sometimes disposed in 
regular meshes, as, for example, in the nuchal sinuses of the 
Rachiglossa. These organs are acquired in the course of ontogeny, 
and are developed in different regions: they are frequently found 
