30 Our Food Mollusks 



isms of typhoid fever and other germ diseases, especially 

 intestinal diseases, from infected waters. 



The organs are exposed when the mantle flaps are 

 lifted, and there are seen to be two of them on each side 

 of the body, one lying nearly over the other. In the 

 figure of Venus, they are represented as being cut off 

 near their bases. In the oyster the gills have been moved 

 from this position on the sides of the body so as to lie 

 in four parallel folds on its ventral margin (Figure 3). 

 Behind the body the four gills unite so as to separate 

 a space above, the cloacal chamber, from the large mantle 

 chamber below. With the unaided eye, it may be seen 

 that each gill is vertically striated. Although at first 

 sight a gill appears to be a solid fold of tissue, closer 

 examination shows it to be made of two plates or 

 lamellae (from which the name Lamellibranchiata, some- 

 times given to the bivalve group, is derived), which en- 

 close a space between them. Each lamella, also, is com- 

 posed of a great number of parallel, hollow rods, the gill 

 filaments, placed regularly, side by side, so that the plate, 

 as represented in a diagram (Figure 4), has a re- 

 semblance to a picket fence. Each filament corresponds 

 to a picket, the lamella to the fence, and the entire gill 

 to two parallel fences. The spaces between the filaments 

 allow water to enter the interior of the gill. 



In one important respect, the illustration of the par- 

 allel fences fails. If we trace a single gill filament from 

 the base of a lamella down to the free lower margin of 

 the gill, we will find that it does not end there, but bends 

 and continues upward as one of the filaments of the other 

 lamella. 



As these rods or filaments are very delicate and much 

 elongated, their regular position might easily become dis- 



