408 MANUAL OF ZOOLOGY. 



walls of the mantle-cavity (as in some of the Heteropoda). 

 Secondly, the respiratory organs may be in the form of out- 

 ward 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 inflection of the mantle. In many 

 members of this last section the water obtains access to the 

 gills by means of a tubular prolongation or folding of the 

 mantle, forming a " siphon" (fig. 214, s), the effete water being 

 expelled by another posterior siphon similarly constructed. 

 In the air-breathing Gasteropods, the breathing organ is in the 



Fig. 214. Ampullaria canaliculate, one of the Apple-shells, o Operculum ; 

 ,y Respiratory siphon. 



form of a pulmonary chamber, formed by an inflection of the 

 mantle, and having a distinct aperture for the admission of air. 



The nervous system in the Gasteropoda has its normal com- 

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

 geal or cerebral, the infra-cesophageal or pedal, and the parieto- 

 splanchnic ; but there is a tendency to the aggregation of these 

 in the neighbourhood of the head. The organs of sense are 

 the two eyes, and auditory capsules placed at the bases of the 

 tentacles, the latter being tactile organs. 



The sexes are mostly distinct, but in some they are united 

 in the same individual. The young, when first hatched, are 

 always provided with an embryonic shell, which in the adult 

 may become concealed in a fold of the mantle, or may be 

 entirely lost. In the branchiate Gasteropods the embryo (fig. 

 215, A) is protected by a small nautiloid shell, within which it 

 can entirely retract itself; and it is enabled to swim freely by 

 means of a ciliated, often lobed extension of the cephalic in- 



