64 Our Food Mollusks 



in cases in which it is placed directly in the mantle cham- 

 ber, a very small amount of it, if any, would ever reach 

 the mouth opening. Moreover, it has been found that 

 clams refuse to allow such substances as very finely 

 ground fish or shrimps even to enter the mantle chamber 

 through the siphons. Mere contact of such materials 

 with the sensory end of the incurrent siphon causes 

 violent contractions of the adductor muscles, and of the 

 siphon also. 



Though the same general plan is followed in all, there 

 are many curious modifications of these ciliary mecha- 

 nisms that are characteristic of different bivalves, and 

 they occur on palps, mantle, or gills. One of the most 

 astonishingly ingenious (if the word may be permitted) 

 of these, is an arrangement on all the gill surfaces of the 

 scallops, by means of which foreign particles, strained 

 from the water, are automatically directed toward the 

 palps and the mouth, when few, and upon outgoing 

 tracts whenever they increase to a certain number. 



To understand the operation of this mechanism, it 

 should be noticed that the gill surfaces are thrown into 

 marked folds, as described in the chapter on anatomy. 

 These folds extend from the bases to the margins of the 

 gills, and between them are deep grooves (Figure 6). 

 The surface cilia of the filaments near the middle of the 

 folds lash toward the free margins of the gills, while 

 those in the grooves between the folds lash in the op- 

 posite direction, toward the gill bases (Figure 16). 

 When the stream that rushes between the filaments into 

 the interior of the gill brings particles to its surface a 

 few at a time, the majority of them fall into the grooves 

 (g), and are then carried to the base of the gill. Here 

 they go to a ciliated tract leading to the palps and 



