MOLLUSCA 241 



The ventral or incurrent siphon opens into the branchial chamber, 

 the dorsal or eoccurrent into a smaller dorsal chamber, the cloacal. 

 Verify and sketch. 



Gill plates or sheets; number and attachment. Are they 

 symmetrical on the two sides? The eggs and developing em- 

 bryos may be found in the outer gill cavity at favorable times. 

 (A special study and report may be profitably made by some 

 student on the structure of the gills as shown by a hand lens 

 and the low power of the microscope. A bit of the living gill 

 from a fresh specimen should be examined.) 



Abdomen, the soft, fleshy mass between the pairs of gills, 

 which terminates in a more solid part, 



Foot : position and form ? 



Mouth and labial palps; at the anterior end and just below 

 the adductor muscle. How many palps ? 



(It is to be remembered that all the structures examined thus 

 far are external organs. The body wall has not been penetrated 

 at all. If it is the plan to study the anatomy more closely, 

 the following are the chief sets of organs deserving attention.) 



4. Other systems of organs. 



Circulatory system. Open the pericardial cavity, just beneath the hinge and 

 a little posterior thereto, find the 



Heart: auricles and ventricle. In a fresh preparation the contractions of 



the heart may be observed. 

 Vessels: one passes in each direction, but they are not easily seen without 



injecting. 

 The intestine passes through the ventricle without open communication 



with it. 

 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 btood 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 dis- 

 section 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 connectives 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. 

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