MOLLUSKS 



2G1 



tough muscles If they are cut lengthwise, the two tubes, incurrent and 

 excurrent, can be easily seen.) The siphon permits water, bearing food 

 m?^^^^f^' ^°rf ^* ^^^^ *^® mantle cavity. Here are found the gills 

 I he Gills. — The gills are striated platelike structures lying on each side 

 of the visceral mass. How many gills on each side? 4ny difference in 

 size of those on one side? When the clam is in a natural position, the 

 gills hang freely in the mantle cavity. In structure each gill is a long narrow 

 bag open on the dorsal side. This baglike opening leads into a second 

 cavity, dorsal to the mantle cavity. This space, called the cloacal cavity 

 is in communication with the outside through the excurrent siphon A 

 mussel when viewed from one end or in cross section somewhat resembles 

 a book. The shell has the position of the board cover, the mantle the 

 paper pasted to its inner surface, the gills the fly leaves, and the bodv 

 the printed pages in the book. 



A fresh- water mussel with the right valve, mantle, and gills and some tissue at the base of 

 the foot removed ; a, anterior adductor muscle ; au, auricle of the heart ; ft, foot ; g, repro- 

 ductive gland ; gl, gill ; i, intestine ; k, kidney ; m, mantle ; n, nerve ganglia connected by 

 nerve cords o ; p, posterior adductor ; pi, labial palps ; si, siphon ; v, ventricle. The dark 

 lines on the mantle and foot are blood vessels. Davison, Zoology. 



Circulation of Water over Gills. — We have already observed 

 that a more or less constant circulation of water takes place; 

 carmine entering through the incurrent siphon passes out through 

 the excurrent siphon. How is this circulation explained? If a 

 sniall piece of the gill of a clam or oyster is placed in a drop of 

 the fluid found in the mantle cavity and examined under the 

 compound microscope, the explanation is found. The surface of 

 the gill is seen to be pierced by numerous holes. These holes, 



