260 MANUAL OP ZOOLOGY. 



and ventricle. Respiration is very variously effected ; one 

 great division (Branchiogasteropoda) being constructed to 

 breathe air by means of water; whilst in another section. 

 (Pulmogasteropoda) the respiration 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 Heteropoda) . Secondly, the respira- 

 tory 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 branchia?, 

 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 Gastero- 

 pods, 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. 



The nervous system in the Gasteropoda has its normal com- 

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

 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 is 

 protected by a small nautiloid shell, within which it can en- 

 tirely retract itself; and it is enabled to swim freely by means 

 of two ciliated lobes arising from the sides of the head ; thus, in 

 many respects, resembling the permanent adult condition of 

 the Pteropoda. 



Shell of the Gasteropoda. The shell of the Gasteropods is 

 composed either of a single piece (univalve), or of a number 

 of plates succeeding one another from before backwards (mul- 

 tivalve). The univalve shell is to be regarded as essentially a 

 cone, the apex of which is more or less oblique. In the 

 simplest form of the shell the conical shape is retained without 

 any alteration, as is seen in the common Limpet (Patella). 

 In the great majority of cases, however, the cone is con- 



