20 Our Food Mollusks 



the oyster lying in its left valve. Here, as in many bi- 

 valves, the margin is provided with projections or ten- 

 tacles, capable of some extension, and acting as sensitive 

 touch organs. Certain parts of this edge are sensitive to 

 changes in the intensity of light, and in the scallop there 

 are developed eyes so perfect in function that moving ob- 

 jects are seen at a distance of several yards. It is evi- 

 dent that this is the only part of the body where visual 

 organs would be of any use. 



In the two forms here illustrated, the mantle folds be- 

 low are separate from each other. In some bivalves, the 

 soft clam for example, there is an extensive fusion of the 

 edges, so that the mantle chamber becomes an enclosed 

 space. 



Venus is a form that spends most of its time burrowed 

 in the bottom just deep enough to cover the shell. As in 

 all other burrowing lamellibranchs, two tubes, the 

 siphons (Figure i), grow out from the mantle posteri- 

 orly, their purpose being to reach up to the water. 

 Through the lower one a stream, bringing food and 

 oxygen, enters the branchial or mantle chamber. After 

 passing through the gills, the water is discharged through 

 the upper tube. These siphon tubes are very long in the 

 soft clam, which burrows many inches into the bottom. 



In addition to these functions, the mantle of forms 

 that possess a large, distensible foot, serves as a blood 

 reservoir, and thus probably functions as the chief organ 

 of respiration; for the blood here is separated from the 

 water only by the very thin mantle walls. 



The Digestive Tract. Referring again to the figure 

 illustrating the hard or round clam, it will be observed 

 that the mantle fold, the two gill folds that hang down on 



