54 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



The gastric mill begins in the hinder half of the cardiac 

 division. Here, on the uj^per wall of the stomach, we see 

 a broad transverse calcified bar (figs. 9-11, c) from 

 the middle of the hinder part of which anotlier bar (?fc), 

 united to the first by a flexible portion, is continued 

 backwards in the middle line. The whole has, therefore, 

 somewhat the shape of a cross-bow. Behind the first- 

 mentioned piece, the dorsal wall of the stomach is folded 

 in, in such a manner as to give rise to a kind of pouch ; 

 and the second piece, or what we may call the handle of 

 the crossbow, lies in the front wall of this pouch. The 

 end of this piece is dense and hard, and its free surface, 

 which looks into the top of the cardiac chamber, is 

 raised into two oval, flattened convex surfaces {t). Con- 

 nected by a transverse joint with the end of the handle 

 of the crossbow, there is another solid bar, which ascends 

 obliquely forwards in the back wall of the pouch (pp). 

 The end which is articulated with the handle of the cross- 

 bow is produced into a strong reddish conical tooth {mt), 

 curved forwards and bifurcated at the summit ; conse- 

 quently, when the cavity of the stomach is inspected from 

 the fore part of the cardiac pouch (fig. 9, B), the two- 

 pointed curved tooth (mt) is seen projecting behind the 

 convex surfaces (t), in the middle line, into the interior 

 of that cavity. The joint which connects the handle of the 

 crossbow with the hinder middle piece is elastic ; hence, 

 if the two are straightened out, they return to their bent dis- 

 position as soon as they are released. The upper end of 



