CRAWFISH 77 



3. A short wide passage, the esophagus, leads upward from 

 the mouth. 



4. The esophagus leads to a large dilated portion of the 

 canal, the stomach. This occupies the main portion of the head 

 region. The larger cardiac chamber lies in front and the smaller 

 pyloric chamber lies behind. The two are separated by a nar- 

 row constriction. 



5. The region of the mouth, esophagus, and the stomach 

 are lined with chitin and are formed by an infolding of the outer 

 skin of the animal as it develops. These parts are collectively 

 known as the stomodaeum. 



6. Posterior to the stomodaeum is the mesenteron. This 

 part lacks the chitinous lining and receives the ducts from the 

 liver. 



7. The proctodaeum, which leads to the anus, is the part of 

 the digestive tube behind the mesenteron. Trace its course. This 

 is lined with chitin, and develops from an infolding of the outer 

 surface. 



8. Make a drawing twice natural size of a side view of the 

 entire digestive system. Label all the parts. 



9. Remove the stomach and carefully examine the gastric 

 mill. Cut longitudinally through the ventral wall and examine 

 the interior after pinning it out expanded. Distinguish the cardiac 

 and pyloric chambers, and the constricted region of the gastric 

 mill. What do you infer to be the main function of this mechan- 

 ism? Notes required. 



10. The cardiac ossicle is a hard plate running across the 

 roof of the cardiac chamber. 



11. The median tooth is at the junction of the pyloric and 

 cardiac chambers. Note the lateral teeth. 



12. Manipulate these teeth to see how they crush the food. 



13. The aperture between the cardiac and pyloric chambers 

 is much narrowed by folds, as is also the entire pyloric chamber. 



