XII.] THE FRESH-WATER CRAYFISH. 127 



anterior edge of the posterior cross-piece. Two powerful 

 muscles are attached to the anterior cross-piece and ascend 

 obliquely forwards to be inserted into the under face of the 

 carapace. Two other similar muscular bundles arise from 

 the posterior cross-piece and, passing obliquely upwards and 

 backwards, are also inserted into the under face of the cara- 

 pace. The disposition of all these parts is such that when 

 these muscles contract, the median tooth moves forwards and 

 downwards, while the lateral teeth move inwards, 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 gas- 

 tric 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 intestine is slender and delicate, smooth 

 internally in the Lobster, papillose in the Crayfish. Near its 

 hinder end its walls become thicker for a short distance, and 

 this thickened portion, with which, in the Lobster, a short 

 dorsal cascum is connected, may be regarded as the large in- 

 testine or rectum. 



The heart is a short, thick, somewhat hexagonal, symmetri- 

 cal organ lodged in the pericardiac sinus, to the walls of which 

 it is attached by fibrous bands. In its anterior half three 



