172 THE MANTLE AND MANTLE CAVITY chap. 



The Mantle 



On the dorsal side of the typical molluscan body, between 

 the visceral sac and the shell, lies a duplicature of the integu- 

 ment, 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 produc- 

 tion 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 pul- 

 monary aperture on the right side. In the Tectibranchiate 

 Opisthobranchs 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 

 altogether. 



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^ Arca}^ tentacles, cilia (^Lima^ Lepton}^ 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 



