FISHES. 199 



(The Fishes.) 



A fish is a cold-blooded vertebrate, adapted for life in 

 water, having the limbs developed as fins, the fingers 

 and toes being represented by cartilaginous rays con- 

 nected by membrane (in rare cases limbs rudimentary or 

 wanting); exoskeleton usually developed as scales or 

 bony plates (skin rarely naked); one or more fins on the 

 median line of the body, composed of rays connected 

 by membrane. Skull developed, containing a brain of 

 several differentiated ganglia; a distinct lower jaw. 

 Heart with an auricle, ventricle, and arterial bulb; 

 respiration by means of branchiae, which consist (typi- 

 cally) "of bony arches attached to the hyoid bone, to 

 which the filaments of the gills are attached, generally 

 in a row upon each, and having their surface covered 

 by a tissue of innumerable blood vessels. The water 

 taken in at the mouth passes among the filaments of the 

 gills and escapes by the gill openings towards the rear; 

 in its progress through the filaments of the gills the 

 water imparts to these the oxygen of the air which it 

 contains. The blood is sent to the gills by the heart, 

 which thus answers to the right side of the heart of 

 warm-blooded animals, and from the gills it is sent to an 

 arterial trunk lying along the under side of the vertebral 

 column, which distributes the blood through the body of 

 the fish" (Guvier); branchiae free, gill openings a single 

 cleft on each side. In most fishes there is a membran- 

 ous air bladder immediately beneath the back -bone, 

 answering homologically to the lungs of the higher 



