vii PRACTICAL DIRECTIONS 391 



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

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

 the large duct of the digestive gland opens into it on either side. Each 

 digestive gland is made up of three main lobes, and consists 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 brain, just 

 behind the bases of the small feelers ; b, the ullet ; 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 

 anus. 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 and the " gastroliths" : 



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 respectively from the median ones men- 

 tioned above, 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 

 hand, and pull gently backwards and forwards (in the direction in 

 which the muscles pull). It will then be seen that the median and 

 lateral teeth come together in the middle line so as to act as a " gastric 

 mill." Note the slit-like lumen of the part of the gizzard behind this 

 and the arrangement of the seta which act as strainers. Make sketches 

 as you proceed. 



