REPTILIA. 87 



duction exhibited in the formation of new limbs in the place 

 of those cut or torn off. ) . * 



GENUS III. PROTEUS, Laur. 



True Amphibia on account of the double use of lungs and 

 branchiae throughout life. 



PISCES. 



Fish are oviparous Vertebratae with a double circulation, and 

 organs of respiration adapted to the element in which they 

 live. The respiration is performed by means of branchice 

 (gills) situated on each side of the neck, composed of nume- 

 rous laminae covered with small blood vessels. The air in 

 the water which passes through these laminae acts upon the 

 blood which is constantly arising from the heart. The heart 

 consists of one auricle and one ventricle which transmit the 

 blood to the branchiae ; from thence it is conveyed into an 

 artery situated under the spine. This vessel acts like the 

 left ventricle in Mammalia and distributes the blood to all 

 parts of the system to be returned by the veins to the heart. 



The structure of Fish is well adapted for swimming, and 

 this is facilitated in a number of species by an air bladder 

 immediately under the spine. The tail is the principal in- 

 strument in progression. The organs analogous to limbs are 

 much reduced and are represented by rays which support 

 membranous fins. Those fins which correspond to the fore 

 limbs are called pectoral, to the hind ones ventral, those situ- 

 ated on the back dorsal, those below anal, and those at the 

 end of the tail caudal The rays are either hard and pointed, 

 consisting of a single piece divided through its length, when 

 they are called spinous; or they consist of a number of pieces 

 articulated together, called articulated rays. 



The head varies in form, but consists of the same number 



