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ZOOLOGY. 



liomology whatever with the like-named structures in 

 Vertebrates. The eight ossicles form a sort of frame work, 

 capable of definite movements by which the three main 

 teeth are brought forcibly together at one point, thereby 

 effectively chewing up the food. The ossicles have been 

 given names the four median ones are, in order from 

 in front back, the cardiac, uro-cardiac, pre-pyloric, and 

 pylwic. Of these the pre-pyloric is doubled back on the 



Fig. 155. GIZZARD OF CRAYFISH. 

 A, median section ; B, dorsal view. 



uro-cardiac, and from the acute angle between them 

 projects the median tooth. The paired ossicles are the 

 small ptero-cardiacs at either end of the cardiac, and the 

 zygo-cardiacs stretching from these to the pyloric and 

 bearing the lateral teeth. The exact arrangement and 

 working of this framework can only be fully understood by 

 a study of the actual thing, though fig. 155 may help the 

 understanding of them a little. As usual, two sets of 

 muscles act in opposite ways on this mechanism : intrinsic 

 muscles, in the wall of the gizzard itself, pull the cardiac 



