112 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 substance 

 of the tiihuliferous portion, and the lobules that form the com- 

 paratively thick walls of the acinous portion. 



2. Find the slit-like 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 muscu- 

 lar 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. Examine the buccal mass. 



(a) The strands of muscles that run from its sides foi'rv'ard 

 to be inserted on the walls of the proboscis are attached to a 

 cartilage, the odontophoral cartilage. These are the cartilage pro- 

 tractors. 



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

 side, are three pairs of slender muscles, one pair median and the 

 others covering the horns of the odontophoral cartilage that has 

 just been observed. Find to what the muscles are attached 

 anteriorly and posteriorly. If the animal is fresh, pull on them 

 with the forceps and see what moves. These are the radula 

 protractors. 



