20 



radiating into the mantle edge wliicli tliey serve to retract 

 and into the suh-epithelial tissue of wliieli they are 

 inserted. They also furnish, the retractor muscle slips 

 wliicli run into the pallial tentacles. The fibres of the 

 internal pallial muscle take origin in the corresponding 

 transverse impression, and form a definite layer of 

 radiating fibres, which are also inserted into the mantle 

 skirt nearer the edge than the level of origin of the other 

 pallial muscles. A continuous ring-nei'^-e runs round in 

 the tissues of the mantle skirt. It is formed in front by 

 the anastomosis of the anterior branches of the pallial 

 nerves, and behind by the fusion of the posterior branches 

 of the same nerves (fig. 24). From the outer side of this 

 ring-nerve numerous twigs are given off, which break up 

 into a network fi'om which branches pass into the pallial 

 tentacles. 



Digestive Organs (figs. 4, 6, 10, 11-22). 



These consist of a very long and complex Gut and 

 some important Glands which open, into it. The mouth 

 opens into a buccal mass (containing the Odontophore), 

 which is succeeded by gullet, crop, stomach and long 

 intestine, the last part of which is the rectum. There 

 are buccal ("salivary") glands opening into the buccal 

 mass and a large digestive gland or hepato-pancreas 

 (" liver ") opening into the stomach. 



I. The Gut. — This complicated tube is formed of many 

 coils arranged in a characteristic manner. We now 

 proceed to the detailed description of its various pa its. 



The Buccal Mass (fig. 6). — This is best exposed by 

 lateral incisions of the anterior part of the body wall. 

 Comiect these by a transverse incision near the tenta(de 

 bases and carefully peel off the dorsal body wall from 

 before backwards, noting that the gullet is very liable to 



