288 MANUAL OF ZOOLOGY. 



sometimes provided with calcareous plates for the trituration 

 of the food. The intestine is long, and its first flexure is 

 commonly "haemal," or towards that side of the body on 

 which the heart is situated ; though in some the flexure is 

 "neural." Distinct salivary glands are usually present, and 

 the liver is well developed. 



A distinct heart is usually present, composed of an auricle 

 and ventricle. Respiration is very variously effected ; one 

 great division (Branchiogasteropoda) being constructed to breathe 

 air by means of water ; whilst in another section (Pulmogas- 

 teropodd) the respiration is aerial. In the former division res- 

 piration may be effected in three ways. Firstly, there may be 

 no specialised respiratory organ, the blood being simply ex- 

 posed to the water in the thin walls of the mantle-cavity (as in 

 some of the Heteropoda). Secondly, the respiratory organs 

 may be in the form of outward processes of the integument, 

 exposed in tufts on the back and sides of the animal (as in the 

 Nudibranchiata], Thirdly, the respiratory organs are in the 

 form of pectinated or plume-like branchiae, contained in a 

 more or less complete branchial chamber formed by an inflec- 

 tion of the mantle. In many members of this last section the 

 water obtains access to the gills by means of a tubular prolon- 

 gation or folding of the mantle, forming a " siphon," the effete 

 water being expelled by another posterior siphon similarly con- 

 structed. In the air-breathing Gasteropods, the breathing- 

 organ is in the form of a pulmonary chamber, formed by an 

 inflection of the mantle, and having a distinct aperture for the 

 admission of air. 



Fig. 102. Ampullaria canaliculata, one of the Apple-shells, o Operculum ; 

 ^ Respiratory siphon. 



The nervous system in the Gasteropoda has its normal com- 

 position of three principal pairs of ganglia, the supra-oesopha- 



