332 MANUAL OF ZOOLOGY. 



water ; whilst in another section (Ptdmogasteropoda) the respi- 

 ration is aerial. In the former division respiration may be 

 effected in three ways. Firstly, there may be no specialised 

 respiratory organ, the blood being simply exposed to the water 

 in the thin walls of the mantle-cavity (as in some of the Hetero- 

 podd). 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 Nudibranchiatd}. 

 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/' the effete water being expelled by 

 another, posterior siphon similarly constructed. In the air- 

 breathing Gasteropods, the breathing organ is in the form of 



Fig. 1-3,4,. Ampullaria canaliculata, one of the Apple-shells, o Operculum ; 

 j Respiratory siphon. 



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-oesophageal 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. 

 135) is protected by a small nautiloid shell, within which it can 



