98 PHYLUM MOLLUSCA 



line ; carry the incision forward in the median line along the head 

 between the base of the tentacles to the mouth. Care should be 

 taken in making this incision not to cut the organs beneath. 

 Spread the flaps as widely as possible to the right and left and 

 pin them down, thus exposing the organs in the forward part of 

 the body. 



The white organs on the right side of the body belong to the 

 reproductive system. The large, dark organ in the center, or 

 on the animal's left, is the stomach. Find the slender, curved 

 oesophagus, which leads forward from it to the dorsal side of the 

 large, muscular pharynx. The oesophagus is encircled by the white 

 nerve collar, the dorsal portion of which is the brain. If, however, 

 the animal died in a retracted condition, the pharynx may have 

 slipped back through the nerve collar, which would then encircle 

 the forward end of that organ. Note the two white, leaflike salivary 

 glands which lie close against the wall of the stomach, and trace 

 their ducts forward to the pharynx. Lying above and across the 

 oesophagus is the white, cylindrical penis, which will be seen to 

 extend from the genital pore at the right of the mouth and to bend 

 sharply on itself. The bend of the penis is connected by a long 

 retractor muscle to the dorsal body wall. Find it ; cut it and pin 

 the penis on the animal's right. Note the broad, glistening re- 

 tractor muscle connecting the pharynx with the ventral, posterior 

 body wall. Notice its shape ; with strong forceps pull it loose from 

 the pharynx and entirely remove it. Beneath it, note the still 

 larger retractor muscle running from the forward end of the head 

 back to the same locality. What is the function of these different 

 retractors ? The dark-colored sheaths and the retractor muscles of 

 the tentacles will also be seen ; trace these muscles to their origin. 

 Find the nerve which passes from the brain into each tentacle. 



Separating the delicate filaments connecting the stomach with 

 the surrounding organs, and pushing it and the oesophagus to the 

 animal's left, find the large nervous mass which forms the ventral 

 portion of the nerve collar, and the nerves radiating from it. It is 

 an agglomeration of ganglia, being made up principally of the 

 pedal and the visceral ganglia. Press the reproductive organs 

 to the animal's right and pin them down. The receptaculum 



