RESPIRATORY AND DIGESTIVE APPARATUS. 



363 



556 ._ PLBUR0AN(:HU8 : m , the mantle 



turned back to show the gills, br : a, the 

 anus ; 6, mouth and proboscis ; v, hood ; 



/.tentacuia; P ,foot. 



in a cavity analogous to that which constitutes the lung of the 

 preceding (Fig. 555) ; but in others, they are placed between 

 the mantle and the foot, or even on the back of the animal, so as 

 to float freely in the surrounding liquid. As examples of the 

 pulmonic Gasteropods, we may mention the Snail, and the Slug, 

 which live on land ; and the 

 Lymnssa, the Planorbis, and 

 others, which live in stagnant 

 waters, and come to the sur- 

 face to take in the air neces- 

 sary for their respiration. 

 Among Gasteropods provided 



with gills inclosed in a dorsal 

 . , n j , v TT i A 



cavity we find the Volutes, 



"Whelks, Cowries, Olives, and 



many others. The Limpets and Pleurobranchi (Fig. 556) have 



these organs in the furrow which separates the foot from the 



mantle ; a nd in 

 the Doris (Fig. 

 568), the Eolis 

 (Fig. 557), and 

 others, they con- 



. 557,-EoLrs. sist of folds or 



tufts, sometimes 

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



904. 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. 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. 



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

 than in the Cephalopods ; the tentacula, which most Gasteropods 



