II.] THE CRAYFISH AND LOBSTER. 1 79 



muscles contract, the dorsal ossicles are divaricated, the uro- 

 cardiac tooth is thereby thrust forwards and downwards, while 

 the lateral teeth move inwards downwards and backwards, 

 and the three meet in the middle line. The working 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 intestine, in both Lobster and Crayfish, is made up of 

 an anterior thin-walled segment whose roof is prolonged up 

 into a coecal process, and a posterior segment which, like 

 the stomach, is lined by a chitinous continuation of the 

 exoskeleton. The latter is spirally folded and papillose in 

 the Crayfish. The anus is bounded by two valve-like 

 thickenings of the exoskeleton, which are connected with 

 the adjacent intestinal wall by a series of small muscles. 

 The alimentary-canal is thus to be resolved into a straight 

 tube of three segments ; an anterior fore-gut and a posterior 

 hind-gut, each lined by an involution of the cuticular exo- 

 skeleton, and a non-cuticular mid-gut which bears the above- 

 named ccecal process and receives the ducts of the digestive 

 gland. The digestive gland itself is the seat of the forma- 

 tion of a combustible carbo-hydrate oily material which is 

 as it were held in reserve in its constituent cells, as well as 



12 — 2 



