VENUS MERCENARIA 7 



When the animal is removed from the bed, the tight fitting 

 valves of the shell are found to be firmly closed. It may be neces- 

 sary to break the shell in order to insert a knife blade by means of 

 which the two powerful muscles which connect the valves, and 

 by their strong contraction close them, may be cut. Removing 

 one half of the shell, it is seen that both shell valves are lined on 

 their inner surfaces by thin, fleshy flaps which grow out from the 

 sides of the body. These are known as the mantle folds [fig. i,m], 

 and they inclose a large space, the mantle or branchial chamber, 

 in which is found the main part of the body. The body, however, 

 does- not entirely fill the mantle chamber, but a large space remains 

 which is filled with water. The siphons are seen to be simply a 

 modified portion of the mantle. It is into this space that the 

 inflowing stream of water, bearing the microscopic food, must 

 enter. The manner in which the food is collected and passed into 

 the mouth will be described presently. While the mantle folds 

 are free at their margins, their edges are closely applied to each 

 other, and the mantle chamber is essentially a closed space, except- 

 ing for the siphonal openings. 



If now one of these mantle folds be cut away, the body is 

 exposed from the side and appears as represented in figure I. 

 The mantle fold on the farther side is shown at m, lining the 

 entire inner surface of the shell valve, s. 



Two large, conspicuous folds, ig and og, the gills, arising from 

 the side of the body, hang free in the mantle chamber. In this 

 position, they are continually bathed by the incoming stream of 

 water, and they perform a very important function in addition to 

 that of the aeration of the blood — that of food collection. Just 

 anterior to the gills, and behind the large anterior adductor muscle, 

 aa, are two small folds, ap and pp, the labial palps. The portion 

 of the palp seen in the figure, ap, is simply the lateral extension of 

 a fold which hangs in front of the mouth like a huge lip drawn out 

 to a point on the sides. The posterior palp is similarly placed 

 behind the mouth. The mouth opening is on the median line 

 behind the anterior adductor muscle, and is hidden from view by 

 the closely applied palps. It is a funnellike entrance to the 

 digestive tract, and, because the food of the clam is microscopi- 



