GENERAL INTRODUCTORY 7 



it has been aerated in the gills, and distributes it 

 over the body through the circulatory system. The 

 latter is peculiar in that its channels are distended 

 in places into wide spaces, or sinuses, which are 

 insinuated among the various organs, and so diminish 

 the body cavity. 



The structure of the gills has been largely employed 

 in the classification of the Mollusca. The gills, 

 which are generally situated in a cavity under the 

 mantle, vary in number from one in the majority of 

 the Gastropoda to eighty pairs in some of the Chitons, 

 or Coat-of-Mail Shells (Polyplacophora). 



Each gill (or " ctenidium ") consists of an axis 

 containing two blood-vessels, one to bring the blood 

 -to the gill, the other to convey it away after it has 

 been aerated in the respiratory filaments. Of these 

 last there is a row on either side of the axis, each 

 filament being more or less flattened. In order to 

 obtain the greatest possible amount of surface 

 exposed to the water for aeration, these filaments 

 are either expanded into leaf-like plates (Aspido- 

 branchiate, Plate III., Fig. 1), as in all the earlier 

 representatives of each of the principal classes, or 

 lengthened out (Pectinibranchiate, Plate III., Fig. 2). 

 Further modifications are dealt with later on. 



The nervous system of the Mollusca acquires 

 peculiar importance in that, while every modification 

 of an organ is reflected in it, it is the last feature to 

 be influenced by these changes, and hence is of 



