GASTROPODA. 



369 



a cellular lens. Pallial eyes* are also found in certain genera of the 

 Chitonidae on the shell pieces ; they possess a calcareous cornea. 



The alimentary canal is usually coiled, and, as a rule, bends 

 forward to open in front on the right side, into the mantle-cavity 

 if that is present. In Chitonidae it opens into the mantle-cavity 

 in the middle line behind, and in 

 some Nudibranchs the anus is median, 

 dorsal, and posterior. 



Most of the more specialized forms 

 possess an invaginable proboscis (Fig. 

 280), the invagination generally begin- 

 ning at the base ; but some possess one 

 which is retractile from the point. 



ill 



FIG. 291. Axial section of the eye 

 of Onchidium (from Pelseneer after 

 Semper). / retina ; II optic nerve ; 

 III pigment; 7F"lens; V cornea. 



In the Natiddae there is a glandular disc 

 on the ventral face of the proboscis, the 

 function of which is to assist the animal in 

 perforating the shells of bivalves. In the 

 Pneumodermatidae the proboscis bears some 

 suckers, which are either isolated or placed 

 upon two retractile lobes (Fig. 321). 



The buccal cavity is provided in its 

 anterior part with a pair of horny 

 jaws, which may be closely approxi- 

 mated dorsally (Natica), or even fused. 

 In Patella, Aegirus, and all Pulmo- 

 nates there is only a single dorsal jaw, and in many forms the 

 jaws are absent entirely. 



The walls of the buccal-cavity are much thickened owing to 

 the muscles of the jaws and odontophore, and to the cartilages 

 of the latter, and give rise to the structure called the buccal mass. 



There is no lower jaw, but on the floor of the buccal-cavity 

 there is a ridge, partly muscular, partly cartilaginous, which has 

 received the same name, though it should rather be regarded 

 as a tongue : this is the odontophore (Fig. 292). The surface 

 of this tongue is covered by a chitinous or horny membrane, 

 called the lingual ribbon, or radula, on which are carried horny 

 teeth of various forms. Behind, the radula passes into a cylin- 

 drical pocket, the so-called sheath of the radula, which projects 

 as a small, generally papilla -like prominence from the ventral 

 and posterior end of the buccal .mass. The radula and its teeth 



* Moseley, H. K, Q. J. M. S., 1884. 



2 B 



