CLASS FOURTH. 
FISHES. 
Vertebhated animals with cold red blood, and respire by 
means of gills or branchiae, and progressing in water by means 
of fins. 
The head in all fishes is large in proportion to the size of the 
body; and although subject to great variety of form, consists of 
the same number of bones. These are separated in young 
fishes, but in adults they become solid, and are then difficult to 
be distinguished. In the head of the perch there are eighty dis¬ 
tinct pieces. The brain is contained in a gelatinous matter, con¬ 
sisting of a number of ganglions. 
The nostrils are simple cavities placed at the front of the 
snout, in the interior of which are radiated laminae. 
The cornea of the eye is much depressed, and has but a small 
proportion of the aqueous humour, but provided with a hard 
globular crystalline lens. They are large in proportion to the 
size of the animal. 
The sac represents the vestibule of the ear, in which are sus¬ 
pended bones of the hardness of flint. There are neither Eus¬ 
tachian tubes nor tympanal bones. It is probable that some 
vibration may be conducted to the head, something analogous to 
sound, but this must be very imperfect. 
The motion of the head is exceedingly limited. In some 
cartilaginous fishes it is attached to the vertebral column by two 
condyles, but this contributes very little to extension or motion. 
