105 



Fishes in general, like the aquatic mollusca, 

 breathe by gills, which consist commonly of 

 four lobes or plates, on each side, of a red co- 

 lour, floating loose in the water at their fringed 

 extremity, but by the other attached to semi- 

 circles of cartilage or bone. These, which are 

 called the thoracic ribs of the animal, are placed 

 on the sides of the upper part of the gullet, and 

 supported by the same bone which supports the 

 tongue, being connected to each other by layers 

 of muscle, by which the gills are kept in con- 

 tinual motion. The water reaches them by the 

 mouth, through four clefts on each side ; and hav- 

 ing passed over the gills, is squeezed out by the 

 descent of the gill-flap, or moveable lid by which 

 the external portion of each gill is covered. The 

 collective surface presented by these fringed 

 lobes, or the part where the blood of the animal 

 comes into contiguity with the water which is to 

 purify it, is enormous so much so, that it has 

 been computed to be, in a fish of the size of a 

 skate, not less than between four and five hun- 

 dred square feet ; and, as all these parts have to 

 be kept in motion in so dense a medium as 

 water, nature has given to fishes extremely large 

 nerves and strong muscles for the purpose. It 

 is not however always by gills, properly so call- 

 ed, that fishes breathe ; some of them, for in- 

 stance the myxine and lamprey, using a different 

 kind of apparatus in respiration. In these ani- 

 mals there are placed on each side of the gullet 

 several small tubes, leading to little vescicles 



