The Crayfish. 4S 



fluid which contains a number of floating corpuscles amoeboid 

 in form. 



The alimentary tract of the crayfish consists of a straight 

 tube, which is not of the same character throughout, and of 

 a pair of large glands, generally called the liver, which open 

 into it. 



The first part of the alimentary canal is the msofhagus, 

 which opens externally by the mouth, and passes upwards from 

 that point to open into the large stomach. The stomach is 

 formed of two compartments, an anterior cardiac and a posterior 

 pyloric portion. The cardiac portion has its inner surface 

 thickened by a number of strong calcified pieces, which together 

 form a masticating apparatus for the animal's food. The 

 various ossicles of this stomachal skeleton are so arranged that 

 three specially hard " teeth,'' borne upon the extremities of 

 some of the ossicles, can be made to converge in the middle 

 line, and effectually break into pieces any hard particle of food. 

 The general arrangenjent of these various ossicles and teeth may 

 be appreciated by an inspection of the accompanying diagram 

 (Fig. 2i); their action can be seen by pulling upon the two 

 ends of the apparatus of the stomach with two forceps. The 

 pyloric part of the stomach is provided with a number of more 

 delicate plates furnished with stiff hairs, the whole forming 

 rather a sifting than a crushing organ. Immediately upon the 

 stomach there follows a very short tract of gut, which differs 

 from the preceding and from the part which follows it, in 

 having soft walls ; it is not lined, as is the rest of the canal, by 

 a thick cuticular lining. This mesenteron is produced dorsally 

 into a short ccBcum, and it receives the ducts of the two 

 hepatic glands. After this comes the hind gut, which has 

 a longitudinally folded cuticular lining; it opens on to the 

 exterior by the anus. 



The excretory organs of the crayfish consist of a single pair 

 of glands known as the green glands ; they are placed in the 

 head region of the body, and open on to the exterior by a pore 

 placed upon the basal joint of the antenna upon a tubercule 

 which that joint bears. The gland, which owes its name to its 

 bright green colour, has been proved to be excretory in function, 



