vn PRACTICAL DIRECTIONS 391 



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

 mid-gut, on the dorsal side of which is a small ccecum. 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 

 gullet ; and c, a pair of white nerve-cords (connectives) 

 coming off from the brain and embracing 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 " gastro- 

 liths " and 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 respectively from the median ones mentioned 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 calcified, 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. 



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