300 INVERTEBRATE MORPHOLOGY. 



and opens into a mouth-cavity frequently provided with two 

 or more chitinous teeth. The pharynx usually receives the 

 ducts of a pair of salivary glands, contains a well-developed 

 radular organ in practically all cases, and communicates 

 posteriorly with a tubular oesophagus. In many cases the 

 anterior portion of the digestive tract is capable of being 

 protruded as a proboscis (Fig. 133, pr), which lies when re- 

 tracted within a proboscis-sheath, formed by a circular infold- 

 ing of the body-wall around the mouth. The intestine (i) is 

 usually more or less coiled, extending into the visceral hump, 

 and presents a stomach-like enlargement which receives 

 the ducts of the digestive gland (dg), a structure usually w r ell 

 developed and forming the greater portion of the visceral 

 hump. The intestine terminates in a straight portion, the 

 rectum (r), which passes forward to the anus (an), which, as 

 has already been indicated, lies in the mantle-cavity, slightly 

 to the right, but occasionally to the left, of the middle 

 line, its position depending upon the amount of rotation 

 which the mantle-cavity and the associated organs have 

 undergone. It should be mentioned that in one suborder of 

 Gasteropods the pericardium and ventricle have wrapped 

 themselves around the rectum in such a way that the diges- 

 tive tube seems to have penetrated the ventricle, a feature 

 which will later be seen to be characteristic of one of the 

 other groups of Mollusca. 



The nervous system has the arrangement which has been 

 described as characteristic of the Mollusca (Fig. 124), the 

 peculiar feature being the crossing of the pleuro-parietal con- 

 nectives which is found in many forms. Numerous modifica- 

 tions of the typical condition are -to be found, consisting 

 principally in (1) the concentration of the ganglia, more 

 especially the cerebral, pleural, and pedal, or the pedal, 

 pleural, parietal, and visceral (Fig. 137), to form a single 

 mass ; (2) in the suppression in some cases of one of the 

 parietal ganglia ; and (3) in the occurrence of several visceral 

 ganglia. In accordance with the flat elongated form of the 

 foot in many species, the nerve-cords passing backward from 

 the pedal ganglia may be of considerable size, and further- 

 more may be connected by regularly-arranged transverse 



