382 MOLLUSCA. 



chosphere, it flattens out and secretes a plate-like shell, which 

 with the development of the visceral sac acquires a cap-like form. 

 At the same time the dorsal integument at the edge of the shell is 

 formed into an annular fold, encircling the visceral sac. This 

 is the mantle-fold; it becomes especially deepened on the right 

 side, and gives rise to the rudiment of the mantle-cavity. The 

 preoral part of the body along the line of the ciliated ring is pro- 

 longed into 2, 4, or 6 processes, and forms the definite velum 

 or sail of the veliger larva. Th foot also becomes much more 

 prominent, and develops in most cases an operculum on its hind 

 end (Fig. 300). 



While these changes are going on, the visceral sac and shell 

 become coiled in a nautiloid fashion, and at the same time more 



developed on the left 

 side than on the right; 

 the beginning of this 

 asymmetry has already 

 been indicated by the 

 development of the 

 mantle-cavity somewhat 

 on the right side of the 

 visceral sac. It soon 

 becomes more marked 

 and results in the 



FIG. 301. Larva of Vermetus (after L. Duthiers). Older mantle-Cavitv COminff to 

 veliger. S velum ; Br gill ; F tentacle ; P foot ; Oc eye. 



lie on the anterior side 



of the visceral sac, and in the commencement of that dextral spiral 

 twist of the visceral sac and shell which is characteristic of so many 

 Gastropods. 



The vascular system arises as a set of spaces in the mesoderm ; it 

 is customary to state that these spaces are persistent remnants of the 

 segmentation-cavity, but it is very doubtful if there is any meaning 

 in this statement. 



The larva often possesses some other special organs which deserve 

 mention : these are the contractile sinuses and the larval kidneys. 

 The former are blister-like projections of the integument, with 

 muscular walls ; they are supposed to assist in the larval circulation, 

 and are found in various parts of the body, e.g., foot. The latter 

 (Fig. 299 d) are a pair of glandular tubes which open externally just 

 outside the velar area on each side. It has been stated that they 

 have internal openings into the blastocoel, but the opposite has also 



