138 MOLLUSCA. 



cartilage so the teeth are turned in. What reason is there for 

 folding the radula in this manner? 



{g) Cut the cartilage protractors and reflect the buccal mass. 

 It is attached to the wall of the proboscis at its posterior end by 

 strong muscles, the radula retractors. These may be studied 

 after cutting the sheath of cross-fibers that hold the mass to- 

 gether. Determine how they are attached to the sides of the 

 radula. Why do they need to be so powerful? 



Make a drawing of the buccal mass as seen from the dorsal side. 



(h) Pull away the muscles and examine the shape of the 

 odontophoral cartilage and its relation to the radula. 



(i) Remove the radula, unfold it, and examine it microscop- 

 ically. Do the teeth differ in any way at the two ends ? Why 

 is the radula so long? 



Draw a portion. 



The radula is the organ upon which most gastropods depend 

 for getting food. You should understand how: 



1. The proboscis is protruded and retracted. 



2. The odontophoral cartilage is protruded and retracted. 



3. The radula is protracted and retracted. By means of a 

 binocular dissecting microscope note its action in a living crep- 

 idula. 



4. The radula is folded by the cartilage and spread for action. 



5. The food is torn off and taken into the mouth. 



Near the base of the proboscis is a pair of large, yellow 

 salivary glands, the ducts from which extend on either side of 

 the esophagus to the mouth. Further back, on the right side 

 of the esophagus, is the small pancreas. 



Trace the esophagus to the stomach and the intestine to the 



anus. 



Nervous System. — Most of the ganglia are grouped around the 

 esophagus, about three-quarters of an inch posterior to the base 

 of the proboscis. They are all brown and accordingly conspic- 

 uous. Carefully cut around its base so the proboscis may be 

 turned back, and the ganglia on the ventral surface of the 

 esophagus may be seen. Carefully pick away the tissue 



