256 CONCHIFERA. 



opening the shells of the Scallop, we see, inside 

 each valve, first a thin and transparent membrane 

 enclosing the entire animal. This is the mantle. 

 Its edges are thickened and surrounded with a fringe 

 of very sensitive fleshy filaments ; they are likewise 

 studded with glands that secrete a colouring matter 

 exactly agreeing with the tints on the exterior of the 

 shell, which they assist in ornamenting. Between 

 the leaves of the mantle are placed the branchiae, or 

 gills, always four in number, formed of fringes 

 pointing outwards, and free at their outer edges, so 

 as to float loosely in the surrounding water. The 

 mouth is placed between the two innermost gills at 

 the point where they unite ; it is a simple orifice, 

 guarded by four thin and sensitive lips. Between 

 the layers of the mantle is enclosed a fleshy organ, 

 something resembling a tongue in its structure ; this, 

 though small in the Scallop, becomes in many genera 

 of large dimensions, and assumes functions of great 

 importance and interest. At the hinge or point 

 where the two shells are united, there is a very 

 elastic substance, the resiliency of which tends to 

 force them apart. To counteract this, a stout, com- 

 pact, and very powerful muscle proceeds from near 



FlG. 194. VENUS CHIONE. 



the centre of one valve to tlyit of the other, which, 

 by its contraction, draws them together, and keeps 

 them closed. When the animal wishes to open its 

 shell, it relaxes this muscle, and the elastic ligament 



