3GO RESPIRATORY AND DIGESTIVE APPARATUS. 



Planorbis, and others, which live in stagnant waters, and come 

 to the surface to take in the air necessary for their respiration. 

 Among Gasteropods provided with gills inclosed in a dorsal 

 cavity we find the Volutes, Whelks, Cowries, Olives, arid 

 many others. The Limpets and Pleurobranchi (Fig. 653) have 

 these organs in the furrow which separates the foot from the 



mantle ; a ad in 

 the Doris (Figl 

 678), the Eolis 

 (Fig. 654), and 

 others, they con- 

 sist of folds or 

 tufts, sometimes 

 very numerous, fixed to the dorsal surface of the body. 



973. The mouth of Gasteropods is surrounded with con- 

 tractile lips, and is sometimes armed with horny teeth, which 

 occupy the palate. In several other animals of this class, the 

 anterior part of the palate is very fleshy, and can be made to 

 project outwards, so as to form a proboscis. The lower part of 

 the mouth and oesophagus is occupied by the tongue, or lingual 

 ribbon, as it is sometimes called, which is covered in most cases 

 with an immense number of minute silicious teeth. This ribbon is 

 usually of great length, sometimes longer than the whole body 

 of the animal ; its anterior part is flattened out, and turned over, 

 so as to enable the teeth upon it to be used ; the posterior part, 

 which bears a reserve of teeth, has its margins rolled together so 

 as to form a sort of tube. The form and arrangement of the teeth 

 upon the lingual ribbon in the Gasteropoda have recently ac- 

 quired great importance in the systematic arrangement of these 

 creatures. In some cases, the stomach also is provided with 

 cartilaginous, or even calcareous, projections or teeth, fitted to 

 divide the food. The intestine is bent upon itself, and is lodged 

 between the lobes of the liver and ovary : lastly, the anus is 

 situated nearly always on the right side of the body, at but 

 a little distance from the head. 



974. In this Class, the organs of sensation are less developed 

 than in the Cephalopods ; the tentacula, which most Gasteropods 



