376 THE CRAYFISH chap. 



cuticle of the gizzard is calcified in places to form the sclerites or 

 so-called " ossicles " of the gastric mill. Note the two niediar» 

 sclerites bounding the transverse constriction in front and-' behind 

 respectively. To theni the anterior and posterior pairs of muscles 

 (P- 375> § S) ^re attached, 



» 2. Following on the gizzard is the short, thin-walled mid-gut, on the 

 dorsal side of which is a small caciim. It has no chitinous lining, and 

 ihe \?ixge dtict of the digestive gland o'pens into it on either side. The 

 digestive gland is made up of three main lobes on either side and con- 

 sists of a number of small blind tubes. 



3. The hind-gut, which runs straight to the anus. Its inner surface 

 is raised into longitudinal ridges which take a slightly spiral course, 

 and it is lined by a thin chitinous cuticle. 



4. Carefully press the gizzard backwards and note a, the hrain, just 

 behind the bases - of the small feelers ; b, the gullet ; and c, a pair of 

 white nerve cords {connectives) coming off from the brain and em- 

 bracing the gullet. Taking care not to injure these parts of the 

 nervous system, cut through the gullet, just above the connectives, and 

 then gently remove the whole enteric canal together with the digestive 

 gland from the body, cutting through the intestine just in front of the 

 Rnus. Examine the whole digestive system under water. 



5. Note again the mid-gut and the digestive glands and ducts ; then 

 remove the digestive gland of one side, and sketch the enteric canal 

 from the same side. Slit up the hind-gut so as to see the ridges and 

 Cuticle. 



6. Clean the walls of the gizzard and note the other sclerites of the 

 gastric mill. 



■ Articulated to each end respectively of the two median sclerites 

 already referred to, in each lateral wall of the gizzard, is a lateral 

 sclerite, the two articulating with one another at their other ends, so 

 that these six sclerites together form a sort of hexagonal frame. Two 

 other median sclerites, arising from the median ones above-mentioned 

 respectively, extend downwards into the constriction between the two 

 portions of the gizzard, and these join below at an angle, where they 

 bear a median tooth. Each -of the posterior lateral sclerites bears a 

 lateral tooth. 



Cut open the anterior end of the gizzard, and note the strongly calci- 

 fied, brownish, median tooth, and the two large lateral teeth. Seize 

 hold of the two median ossicles with two pairs of forceps, one in each 



