FISHES 133 



formidable weapons. In some of the catfishes poison 

 glands in addition are provided, and an extremely 

 painful poison is introduced with each prick of the 

 spines. The porcupine-fish, which has no scales, is 

 covered with long quill-like spines; the sturgeons 

 bear long rows of bony plates; the trunk-fish is 

 incased in a bony box; and the sharks have an ex- 

 tremely tough skin covered with shagreen. 



The fins, as stated in our definition of a fish, take 

 the place of the limbs in higher vertebrates and 

 serve a similar purpose, for they are the organs by 

 means of which the fish balances and propels itself 

 in the water. The fins are commonly seven in num- 

 ber. Three fins are single and in a vertical plane, 

 and the other four are paired. One pair is attached 

 to the shoulder girdle and corresponds to the fore 

 limbs of mammals; the other pair is attached to 

 some sort of pelvic girdle and corresponds to the 

 hind limbs. 



The herrings and many other fish have a single 

 fin on the back. Others like the mullets have two 

 such fins. When two such fins are present, the an- 

 terior one generally is composed of spines. The cod- 

 fishes, however, have three fins on the back and two 

 below the body, behind the vent, all composed of 

 soft rays. The mackerels differ from other fish in 

 having a whole series of small fins both above and 

 below the tail. The skates and rays, which live and 

 feed on the floor of the waters they inhabit, have no 

 fin underneath the body, probably because such a 

 fin would be an obstacle subject to injury, rather 

 than an aid. The shoulder fins are very small or 



