GENERAL NOTES ON MOLLUSCS. 



413 



the sedentary oyster among Lamellibranchs ; in the pelagic 

 Pteropods part of it forms lateral wing-like lobes used in 

 swimming, while in lanthina, which has a similar habit, its 

 chief use is to secrete a " float " to which the egg-capsules 

 are attached. In various Lamellibranchs, and in Dentalium^ 

 it is modified as a conical boring organ. 



The mantle is another important Molluscan structure, 

 and as it secretes trie shell, the shape of the latter is of 

 course determined by it. Primitively the mantle is repre- 

 sented by a uniform downgrowth of skin from the dorsal 

 surface, surrounding the ventral foot, and secreting a dorsal 

 cap-shaped shell. Such a simple condition occurs in the 

 limpet. In the Lamellibranchs, with the lateral flattening 



FIG. 223. Bivalve (Panopaea norvegica\ showing siphons. 

 *., Exhalant aperture ; z'., inhalant aperture. 



of the body, the mantle becomes divided into right and left 

 halves, and the shell becomes two-valved. In most Lamelli- 

 branchs the mantle is prolonged into two tubes or siphons, 

 through which the water of respiration enters and leaves the 

 mantle cavity. A similar but unpaired siphon is found in 

 many Gasteropods. In Scaphopoda the mantle folds fuse 

 ventrally to form a continuous tube. In most Gasteropods 

 the mantle skirt is retained, and secretes a spiral shell, as 

 well as enclosing a space in which the gills lie ; in 

 some, both mantle and shell are absent. In the snail 

 and its allies (Pulmonata), the mantle forms the 

 pulmonary chamber, which opens to the exterior by a 

 small aperture. In Cephalopoda the mantle skirt is well 

 developed and muscular, and, besides sheltering the gills, is 

 of much importance in locomotion. 



