2/4 ELEMENTARY BIOLOGY. [CHAP. 



gather with the wall of the pulmonary chamber which over- 

 lies it in front, is invested in the spirally coiled shell, the 

 apex of which lies altogether to the animal's right side. 

 The mouth or peris tome of the shell overlies the thickened 

 anterior border of the pulmonary sac, from which a constant 

 addition of shelly matter is secreted during the growth of 

 the animal, as is also the hybernaculum during repose. 



The aperture of the mouth is bounded by soft fleshy lips, 

 and it leads into a spacious buccal cavity the walls of which 

 are excessively thick and muscular. A denticulate horny 

 upper jaw or beak is present, and the floor of the mouth is 

 raised up into a cushion-shaped odontophore or tongue, 

 which is in turn surmounted by a dentigerous lingual-ribbon 

 or radula. This is thrown into a licking rasp-like motion 

 during feeding, by the activity of an underlying musculo- 

 skeletal apparatus, the odontophoral cartilages connected 

 with which are worthy of note as composing an endoskeleton. 

 The mouth itself leads into a long tubular oesophagus, 

 which passes straight back and, on entering the visceral 

 sac, opens into a small stomach which receives the secretion 

 of the digestive gland. The stomach in turn gives origin 

 to a coiled intestine which, on nearing the exterior, skirts 

 the lower right-hand border of the pulmonary sac, termi- 

 nating in an anus which lies to the right of the respiratory 

 orifice. The middle segment of the oesophagus is enlarged 

 to form a distensible crop, applied to which there are a 

 pair of salivary glands, confluent above and pouring their 

 secretion into the mouth by means of two elongated 

 ducts. 



The digestive gland is a paired structure ; its lobes are 

 asymmetrical the smaller right one lying altogether within 

 the top whorls of the shell. Microchemical examination 

 shows that it performs a complex function, serving both as a 



