58 A LOBSTER OR A CRAYFISH. 



g) Immediately beneath the dorsal abdominal artery lies the intestine, 

 leading from the stomach to the anus. 



h) The large flexor muscle fills most of the space within the abdomen. 

 Why is it more strongly developed than the extensor muscle ? 



i) On both sides of the stomach and partly beneath and behind it is a 

 pair of digestive glands whose secretions are poured into the 

 alimentary canal by a common duct at a point near the junction of 

 intestine and stomach. 



j) Situated beneath the heart and in front of and behind it are the 

 reproductive organs. In the female crayfish the ovaries are 

 brownish yellow, in the male the testes are white. From the latter 

 organs the coiled sperm ducts lead to the external openings pre- 

 viously mentioned. The external openings of the oviducts have 

 also been previously noted. 



Exercise 5. Make an outline drawing, on the same scale as in exercise i, 

 showing the above-named organs in position. 



Technical Note. Clip off the (esophagus close to the stomach, and the 

 sternal artery close to the heart. Remove carefully all the organs 

 -figured in Exercise 5, noting in the process any details that could not be 

 observed before. Place the stomach and the heart in water. Pick 

 away all muscles and other tissues covering the ventral nervous 

 system. 



k) Note six openings in the heart, two on the dorsal surface, two on the 

 ventral and one on each side. These are the ostia ; by means of 

 their valves blood is admitted to the heart from the surrounding 

 pericardium, a membranous sack which was probably torn away 

 in removing the carapace. 



1) Sit open the stomach and note the chitinous lining, with its toothed 

 Iprojections forming what is called the gastric mill. This lining 

 is shed with the outer exoskeleton in moulting. 



m) Situated in the head, below the bases of the antennas is a pair 

 of excretory organs, the green glands. The external openings 

 of these glands have already been pointed out. 



