13* ELEMENTARY BIOLOGY. [CHAP. 



below the pyloric piece. It is connected with this, however, 

 by a short ' pre-pyloric ' ossicle which ascends obliquely 

 forwards and is articulated with the anterior edge of the 

 pyloric piece. The lower extremity of this is produced into 

 the strong median 'uro-cardiac' tooth; two small 'cardiac' 

 teeth are borne by the median process of the cardiac ossicle; 

 while the postero-lateral pieces are flanged inwards, and, 

 becoming greatly thickened and ridged, form the large 

 ' lateral cardiac ' teeth. Two powerful muscles are attached 

 to the cardiac ossicle, and ascend obliquely forwards 

 to be inserted into the under face of the carapace. 

 Two other similar muscular bundles arise from the pyloric 

 ossicle, and, passing obliquely upwards and backwards, are 

 also inserted into the under face of the carapace. The dis- 

 position of all these parts is such that when these muscles 

 contract, the uro-cardiac tooth moves forwards and down- 

 wards, while the lateral teeth move inwards downwards and 

 backwards, and the three meet in the middle line. The 

 action of these muscles can be readily imitated by seizing 

 the anterior and posterior cross-pieces with forceps and 

 pulling them in the direction in which the muscles act. The 

 three teeth will then be seen to come together with a clash. 

 Thus the food which has been torn by the jaws is submitted 

 to further crushing in this gastric mill. The walls of the 

 pyloric division of the stomach are thick, and project like 

 cushions into its interior, thereby reducing its cavity to a 

 narrow passage. The cushion-like surfaces of the pyloric 

 walls are provided with long hairs which stretch across this 

 narrow passage, and thus convert it into a strainer, which 

 allows of the passage of only very finely divided matter from 

 the gastric sac to the thin and delicate intestine. The 

 hepatic ducts open, one on each side, at the junction of the 

 pyloric division of the stomach with the intestine. The 



