Introduction to Animal Morphology. 287 



the digestive canal. This is protruded by its circular 

 fibres, and by means of its lacunae becoming distended 

 with blood ; it is retracted by muscles attached not to 

 its tip alone, as in the proboscis of worms, but to its 

 whole extent. In some the proboscis is permanently 

 partly protruded. A similar proboscis often exists in 

 Heteropoda. The mouth is anterior, may be bordered 

 by lips,* with a marginal sphincter, and leads into a 

 buccal cavity, surrounded by a reddish muscular wall 

 (sometimes with transversely striped fibres), forming 

 an oval, buccal mass, in which are imbedded two or 

 four true cartilages, forming an early example of an 

 endo-skeleton. To these the muscles are attached, 

 and they support the masticatory organs. These 

 may be horizontally opposed lateral jaws armed with 

 flat plates (Heteropoda, Paludina), or sharp teeth 

 (Dolium), sometimes approximated (Marsenia) or 

 united above, forming a vertical unpaired jaw on the 

 upper pharyngeal wall, as in Pulmonata ; but more 

 commonly there is an inferior, unpaired, median organ 

 [radiila, odontophore, or tongueX) on a cartilaginous 

 support, bearing on its surface teeth arranged in suc- 

 cessive rows, each consisting of a middle or axial 

 tooth (rachis teeth, denies), bordered by one or more 

 intermediate teeth [uncini) on each side, outside 

 which are lateral teeth. The radula begins posteriorly 

 in a sacij: in the pharynx below the oesophagus, and is 

 partially retracted thereinto. When not in use it 



* The lips are fringed in some, as in Nubecula tulipa. 



t The tongue proper is the eminence on the floor of the mouth sup- 

 porting the radula, and containing one or two cartilages ; it has a central 

 ca\-ity (ca\-um pulvinare), and a retractor muscle from the cartilage. 



X The tongue sheath is variable, spirally wound and long in Patella. 



