THE CLAM 127 



The nacreous layer is secreted by the surfaces of the body 

 and of the mantle in contact with the shell. It may con- 

 tinue to form throughout life. Is there any proof that this 

 has happened? 



4. Body. While working on the following external features 

 of the body remember that while the shell is properly a 

 part of the body, an exoskeleton, yet the fleshy part exposed 

 by the removal of the shell is still the external portion of 

 the body of the animal. 



(a) Mantle. This is the thin membrane lining the shell 

 and covering the rest of the body. How many lobes are 

 there? Are they attached to the shell? If so, where, and 

 to what extent? Are they joined to each other at any point? 

 In the posterior part of the body observe that the margins 

 of the mantle are hollowed out to form two oval openings, 

 a ventral incurrent, and a dorsal excurrent opening or siphon 

 (in Venus the mantle is fused to form two tubes). Deter- 

 mine the cavities into which the openings lead. All the 

 water that comes into the shell enters through the incurrent 

 and all leaves by the excurrent opening. The cavity be- 

 tween the mantle lobes is the mantle cavity and in it are 

 several organs. 



(6) Gills. Remove or turn back the mantle from one 

 side and expose the gills, which are thin membranes hang- 

 ing freely in the mantle cavity. How many on each side 

 of the body? Are they of similar size and form? How 

 and where are they attached to the body? In the female 

 the gill acts as a brood pouch during the breeding season 

 and is then greatly distended by the contained embryos. 

 A study of sections of the gill may be made to show the 

 layers of which the gill is composed, the water tubes, and 

 the bloodvessels. 



