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 Janthina, 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 Dextalium, 
itis modified as a conical boring organ. 
The mantle is another important Molluscan structure, 
and as it secretes the 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 
Fic. 223.—Bivalve (Panopea norvegica), showing siphons. 
e., 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. 
