MANTLE IN MOLLUSCS 



467 



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 Den- 

 talium, it is 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 

 Umpet. In the Lamellibranchs, with the lateral flattening 

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

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



FiG. 265. — Bivalve (Panopcea norvegica), showing siphons. 

 e., Exhalant aperture ; ('., inhalant aperture. 



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 (P.ulmonata), 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. 



Typically the Mollusca are bilaterally symmetrical 

 animals, and this symmetry is marked in the Solenogastres 

 and Lamellibranchiata, and occurs to a less extent in the 



