MOLLUSCA 243 



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 dissection. Trace 



the intestine through the abdominal mass, and plot its course. Identify 



the stomach, the esophagus, 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, 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. 



280. The Oyster.^One or two students should prepare a report on the struc- 

 ture 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? 



281. The Pond Snail {LimncBo). 



1. The Living Animal. — Observe, both in its natural home 

 and in glass vessel containing water, in the laboratory. 



To what does the animal adhere in the water ? Must it have 

 solid support? Can it swim? What is its method of locomo- 

 tion ? What does it eat, and how ? Can you determine whether 

 it uses the air in breathing or gets its oxygen from the water ? 

 Proof? How is the gliding motion effected? Watch, with a 

 lens, one crawling along the side of the glass vessel. Record 

 signs of sensitiveness to stimuli, by experiments of your own 

 devising. 



2. General Form. — Is there any sign of bilateral symmetry? 

 In what parts ? How are anterior and posterior distinguished ? 

 Relation of the shell to the animal ? Identify : — 



Head: tentacles, number and position; eyes, number and 

 position. 



Foot, the muscular expansion: shape, changes in form and 

 position. 



