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purpose, like those in leaves, have hitherto been 

 detected in them. Perhaps the simplest form in 

 which a distinct respiratory apparatus displays 

 itself in animals, is that of gills, such as are met 

 with in most of the aquatic mollusca, as the 

 fresh-water mussel and the cuttle. In the former 

 of these are three pairs of gills, one about the 

 mouth, and two running down on the sides of 

 the body the one exterior and the other inte- 

 rior. They lie within what is called the cloak of 

 the animal, a thin membrane with a fissure at its 

 posterior part to admit the water, charged with 

 air, to the surface of the gills ; and this water, 

 after having performed its office in purifying the 

 blood which circulates through the gills, is 

 squeezed out again by the closing of the shell. 

 The cuttle has one pair only of leaf-shaped fring- 

 ed gills, lying also within its cloak, to the inner 

 surface of which they are tied down by ligaments, 

 and by an opening in which the water is admit- 

 ted ; while it is afterwards squeezed out by the 

 contractions of this cloak the cuttle having no 

 investing shell which is accordingly thick and 

 fleshy for the purpose. Another form of respi- 

 ratory apparatus is met with in the terrestrial 

 mollusca, which respire air and not water. This 

 in the common garden snail occurs in the form 

 of a bag placed immediately opposite to the up- 

 per part of the shell, and opening at the aperture 

 of the shell, towards the right side ; the ingress 

 and egress of the air being regulated by a proper 

 muscle which surrounds the opening. The vivi- 



