THE GASTROPODA 



135 



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 

 (Purpurd). 



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, 



in 



KIG. 114. 



Embryo of Vermctus, ventral 

 aspect. I, velum ; II, contractile 

 sinus ("embryonic heart"); III, 

 opening of the pallial cavity ; IV, 

 shell ; V, foot ; VI, left eye. (After 

 Salensky.) 



FIG. 115. 



Larval shell of Nasso. rrticiilata, 

 ventral aspect, x 30. h, hook of 

 the dorsal edge of the aperture ; si, 

 future canal or shell-siphon ftp, 

 spire. 



or is 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, Ituncina, 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 



