148 MOLLUSCA 



already been noticed. Cut along their common line of union 

 and examine the inner surface of each part. 



1. Notice the parallel lines of tubules that form the sub- 

 stance of the tubuliferous portion, and the lobules that form 

 the comparatively thick walls of the acinous portion. 



2. Find the slitlike opening that leads from the kidney to 

 the mantle cavity. It is at a point between the two portions 

 of the kidney and is easily found from the mantle chamber. 

 A small opening leads into the pericardium, but it is hard to 

 find it in dissections. 



Digestive System. — 1. Remove part of the integument at 

 the base of the proboscis and find the muscles that retract it. 

 How many are there and how are they attached? Do you 

 understand how the proboscis is extended? 



2. With a pair of scissors open the extended proboscis 

 along the ventral line, pin it open, and notice that the exposed 

 muscular mass, the buccal mass, is attached to the wall of the 

 proboscis in the region of the mouth, at its base, and by 

 means of fibers along its sides. 



3. Push the muscular mass slightly to one side and notice 

 the esophagus, which is closely applied to the dorsal wall of 

 the proboscis. Notice the muscle fibers that extend from it 

 to the proboscis. What is their function? 



4. The odontophoral apparatus consists of a forked car- 

 tilage, the odontophoral cartilage, that is surrounded by 

 muscles and cannot be seen until these are removed, a radula 

 which is for most of its length enclosed in a sac, the radular 

 sac, and is exposed only in the region of the mouth, and the 

 muscles for moving the cartilage and the radula. 



(a) The strands of muscle which run forward from the 

 odontophoral cartilage to be inserted on the walls of the pro- 

 boscis are the cartilage protractors. 



(b) Attached to the ends of the two horns of the cartilage 

 and running posteriorly to be attached to the walls of the 

 proboscis near its base are the flat cartilage retractors. 



(c) Running lengthwise of the buccal mass, on its ventral 



