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EVOLUTION OF ANIMALS 



Part V 



Fig. 31.5. Locomotion of the fresh-water clam is slow as in most clams except- 

 ing the razor shells. Blood is forced into the foot and it reaches forward. This takes 

 time. Finally, the muscles of the foot contract and pull the body forward. Thus, the 

 clam takes a step. (Reprinted from Animals Without Backbones by Buchsbaum by 

 permission of The University of Chicago Press. Copyright 1948.) 



and freely movable for a short distance back to the pallial or fence line where 

 the mantle is attached to the shell. In the soft-shell clam (Mya), the borders 

 of the right and left folds of the mantle are grown together and form the band 

 of flesh prominent in steamed clams. At the rear, usually recognizable by the 

 more pointed end of the shell, the flaps of the mantle are joined and form a 

 tube with fleshy walls. This contains the siphons. In some clams, there are two 

 tubes but if single, the tube is divided within by a partition. Drawn in by the 

 cilia on the mantle and gills, water passes into the incurrent or ventral siphon 

 carrying microorganisms and other particles of food with it (Fig. 31.7). Part 

 of the water is carried toward the mouth and part of it enters the gills. After 

 passing through the gills it passes out the excurrent or dorsal siphon taking 

 away metabolic waste. Although always at the rear end, the siphon is com- 

 monly called the neck, long neck for the soft shelled Mya, little necks for the 

 round clams. The tips of siphons are heavily pigmented and black, removed 

 as inedible for indoor meals, eaten with relish at outdoor parties (Fig. 31.8). 

 Shell. The shell is composed of three layers; the outermost or periostracum 

 is thin, often horny; the middle one contains prisms of lime (calcium carbo- 

 nate), and the innermost pearly layer is composed of crystals of lime lying 

 irregularly parallel to the surface so that they break up the rays of light and 

 create iridescence (Fig. 3 1 .6) . The pearly layer is secreted by cells in the whole 



