CRAYFISH 85 



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

 entire digestive system. Label all the parts. 



9. Remove the stomach. Cut longitudinally through the 

 ventral Avail and examine the interior after pinning it out ex- 

 panded. Examine the hard, tooth-like structures which make up 

 the gastric mill. Distinguish the cardiac and pyloric chambers, 

 and the constricted region of the gastric mill. What do you in- 

 fer to be the main function of this mechanism? Note 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. 

 These folds are fringed with hairs so that they form a strainer 

 or filter which prevents the passage of large food particles into 

 the intestine. Make drawing of gastric mill as dissected. 



14. Manipulate the mandibles and learn where the large 

 muscles are attached. Distinguish the tendons. 



Nervous System 



1. The central nervous system consists of a chain of paired 

 ganglia extending the length of the body close to the mid-ventral 

 line. The ganglia of a pair are so closely applied that they ap- 

 pear as a single ganglionic mass. Locate the nerve chain near 

 the base of the abdomen. Trace it forward along the floor of 

 the thorax to a point where it enters a small canal. With a pair 

 of forceps pick away the parts of the endophragmal skeleton 

 which form the canal surrounding the nerve chain. 



2. The brain is a rather large white body just behind and 

 above the bases of the antennules. This supplies nerves to the 

 eyes, antennae, and antennules. 



