GENERAL INTRODUCTORY 9 
it has been aérated 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 thegills 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 aérated 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 aération, 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 IIT., 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 
