1/2 THE MANTLE AND MANTLE CAVITY chap. 



The Mantle 



On the dorsal side of the typical niolluscan body, between the 

 visceral sac and the shell, lies a duplicature of the integument, 

 generally known as the mantle. The depending sides of the 

 mantle, which are usually somewhat thickened, enclose between 

 themselves and the body mass a chamber of varying size and 

 shape, called the mantle cavity, which communicates freely with 

 the external air or water, and encloses and furnishes a protection 

 for the organ or organs of respiration. On its upper or dorsal 

 surface the mantle is closely applied to the shell throughout its 

 whole extent, the cells with which it is furnished secreting the 

 materials from which the shell is formed (see p. 255). The whole 

 mantle is capable, to some degree, of secreting shelly matter, but 

 the most active agent in its production is the mantle edge or 

 margin. 



In the Prosobranchiata the mantle cavity, for reasons which 

 have already been explained, is found on the left side of the 

 animal, its front portion being in many cases produced into a 

 tubular siphon. Within the mantle cavity are found, besides the 

 branchia, the anus, the apertures of the kidneys, and the 

 osphradium. In the Pulmonata the mantle fold encloses a so- 

 called lung-cavity. The front edge of the mantle coalesces with 

 the integument of the neck in such a way as to enclose the 

 cavity very completely, the only communication with the outer 

 air being by means of the contractile breathing or pulmonary 

 aperture on the right side. In the Tectibranchiate Opistho- 

 branchs the mantle fold is inconsiderable, and is usually 

 not of sufficient extent to cover the branchia, while in the 

 Nudibranchs, which have no true branchiae, it disappears 

 altosrether. 



In the Pelecypoda the mantle cavity is equally developed on each 

 side, enclosing the two sets of branchiae. The mantle may thus 

 be regarded as consisting of two equal portions, which form a sort 

 of lining to the two valves. The lower or ventral portion of the 

 mantle edges may be simple, or provided with ocelli {Pecten, Area), 

 tentacles, cilia {Lima, Lejpton), or doubled folds. The two portions 

 of the mantle touch one another along the whole line of the 

 edge of the two valves, and, although thus in contact, may remain 

 completely separate from one another, or else become permanently 



