Anatomy of the Food Mollusks 29 



extended in another direction. The process is repeated 

 until several threads are formed. Though the mussel 

 remains attached during its life, it is possible for it to 

 cast off the byssus at its base and form a new one. The 

 scallop has the same habit, but attaches less frequently 

 after attaining the adult condition. In the clams the 

 byssus disappears early in life. 



Still another use is made of the foot in a few cases. 

 The large sea-clam (Mactra) of the north Atlantic 

 coast, is able to leap a distance of several inches, when 

 out of the water, by a quick movement of the organ, 

 and it is not difficult to see how such a habit may, at 

 times, be useful to it. There are some cases in which 

 bivalves are able even to swim short distances by a rapid 

 paddle-like movement of the foot. 



Finally it is to be noticed that in the oyster, the foot, 

 although present in the very young, early begins to dis- 

 appear, and soon completely vanishes. This is presum- 

 ably the result of the mode of life inaugurated by the 

 attachment of the shell to some object on the bottom, for 

 the foot is then of no use. 



The Gills or Branchiae. The gills are the most 

 complicated organs of the lamellibranch body. They, 

 like the foot, have been greatly changed from the primi- 

 tive condition to conform to various modes of life which 

 characterize different species. So many of the activities 

 of bivalves depend on them, and so much also concern- 

 ing their functions is of importance to those who use 

 oysters and clams for food, that they should be thor- 

 oughly understood. No one, for example, who knows 

 how these organs continually strain from the water the 

 minutest solid particles, and hold them tenaciously, can 

 have any doubt about their power to collect the organ- 



