MOLLUSCA. 237 



Excretory organ. 



Organ of Bojanus, or kidney, lies just beneath the floor of the 



pericardial cavity, one part on either side. 



Each portion is a dark-colored sac, with an abundant blood supply. 

 Nervous system. (Traced best in hardened preparations.) 

 Visceral ganglia. Look between the gills in the posterior portion of 

 the body, beneath the posterior adductor muscle, and in the floor 

 of the cloacal cavity. Number, and closeness of connection? By 

 careful dissection determine what nerves leave them. Trace a 

 pair of these forward to the 



Cerebral ganglia, on either side the mouth. Note the connections 

 between the cerebral ganglia. Trace from these ganglia the con- 

 nectives which pass ventrally to the 

 Pedal ganglia in the muscular foot, close to its union with the 



abdomen. 

 Make a clear diagram showing the relations of these three pairs of 



ganglia. 



Digestive system. 



Begin with the intestine at the heart. Trace posteriorly to the anus. 

 What is its relation to the posterior adductor muscle? Pass a 

 bristle into the intestine anteriorly and use it to guide the dissec- 

 tion. Trace the intestine through the abdominal mass, and plot its 

 course. Identify the stomach, the oesophagus, and the mouth. The 

 liver is a brownish or greenish mass surrounding the stomach. 

 Much of the visceral mass through which the intestine coils is made 

 up of the large reproductive glands which open into the mantle 

 cavity. 



5. Cross Sections. A series of cross-sections may be made by the 

 teacher, numbered, and used with profit as demonstrations. For such 

 sections the soft parts of the animal should be hardened for 24 hours in 

 i per cent, chromic acid ; then one day each in 70 per cent, and 90 per cent, 

 alcohol. Keep in 95 per cent, alcohol for a few weeks. Cut one fourth 

 to one third inch thick and number so as to be able to locate position 

 of section. Float in dish of alcohol and identify the parts found. Make 

 sketches of sections passing (i) through the stomach, (2) through the 

 heart, and (3) through the middle of the posterior adductor muscle. In 

 the absence of these the student should be encouraged to make a diagram 

 of an imaginary cross-section through the middle of the body. Include 

 the shell. 



274. The Oyster. One or two students should be asked to prepare a 

 report on the structure of the oyster and present to the class an account 

 of the chief points of contrast between the oyster and the clam. The adult 

 oyster is fixed by one of its valves. Is it the same one in all specimens ? 



275. The Pond Snail (Liinn&a). 



i. The Living Animal. Observe, both in its natural home 

 and in glass vessel containing water in the laboratory. 



