244 MOLLUSCA. 



Pecten^\ the Scallop. 



On opening the shell, we see inside each valve, 

 first, a thin and semitransparent membrane, united 

 to each other, near the hinge. This is the mantle. 

 Its edges are thickened, and surrounded with a 

 delicate fringe of very sensible fleshy filaments : they 

 are studded with glands, which in many cases se- 

 crete colouring matter agreeing with the tints on the 

 shell: the shell being enlarged by these glandular 

 fringes, much in the same way as we have described 

 in the Murex. Between the two leaves of the mantle 

 are placed the lungs or gills, four in number, com- 

 posed of fibres pointing outwards, of exquisite struc- 

 ture, free at their outer edges, so as to float loosely in 

 the water. The mouth is placed between the two 

 inmost gills, where they unite ; it is a simple orifice, 

 without anything answering to teeth, but having 

 four thin membranous lips on each side the aper- 

 ture. Near the mantle is placed a fleshy organ, 

 somewhat resembling a finger, capable of enlarge- 

 ment and contraction, called the foot, which, though 

 small in the Scallop, becomes, in some genera, of 

 large dimensions, and assumes functions most impor- 

 tant and interesting. 



Two circumstances here are likely to strike an 

 attentive inquirer, and to awaken his curiosity. 

 Confined as the animal is, to an almost motionless 

 shell, how is it provided with that constant succes- 

 sion of fresh and unre spired water to the gills, 

 * Pecten, a comb. 



