CLASS PISCES. 



79 



portion is tlie " backbone," which is seen to form a long 

 axis or central stem, round which the body is symmetrically 



Fi<, ,, —Skeleton of the common Perch {Perca fiuviatiUs). p Pectoral 

 °'fin- V One of the ventral fins; a Anal fin, supported upon mterspmous 

 lioiies (0 ; c Caudal fin ; d First dorsal fin ; d' Second dorsal fin, both sup- 

 ported upon interspiuous bones ; * i Interspinous bones ; r Ribs. 



built up. The backbone is technically called the " ver- 

 tebral column," and is composed of a succession of short 

 pieces or segments, each of which is termed a "vertebra" 

 (Latin, verto, I turn). Each separate piece or vertebra of 

 the backbone is deeply cupped or hollowed out at each end, 

 and the entire spine is thus rendered extremely flexible. 

 From the possession of this backbone or spinal column, 

 the Perch is said to be a " vertebrate animal." The essen- 

 tial purpose served by the backbone is to protect the 

 very important portion of the nervous system which is 

 known as the "spinal marrow," or "spinal cord." The 

 unpaired fins are all connected with the backbone by 

 means of a series of little bony spines, which are placed 

 in the central line of the body, and are known as the 

 " interspinous bones " (fig. 35, i). 



At the front end of the backbone is placed the skull, 

 which protects in its interior the brain or central portion 

 of the nervous system. 



The mouth in the Perch is placed at the front of the 



