MABINE VERTEBRATES 311 



down, but sometimes they are persistent structures more or less 

 embedded in the bone of the jaws. In some cases teeth are alto- 

 gether wanting, but in others they are situated not only on the 

 jaws, but also on the tongue, the roof of the mouth, and even on 

 the bony arches that support the gills. 



A glance at the fins of a typical fish will suffice to show that 

 they may be divided into two groups the paired fins, representing 

 the two pairs of limbs in most of the higher animals, and the median 

 fins occupying the middle line of the body. The former comprise 

 the two pectoral fins that correspond with our arms, and are 

 attached to the bones of the pectoral or shoulder girdle ; and the 

 pelvic fins, corresponding with the lower extremities. The pectorals, 

 too, are present in nearly all fishes, while the pelvic pair are rather 

 more frequently absent than the pectorals. 



The medial fins comprise the dorsal, the ventral, and the caudal 

 or tail-fin, and are not to be regarded as limbs, but rather mere 

 outgrowths of the skin. They are not directly connected with any 

 part of the main bony framework of the body, though they are 

 generally jointed with a series of bones (interspinal bones) that 

 run between processes of the vertebral column. The dorsal and 

 ventral fins are often divided into two or more parts, and the tail 

 fin is commonly distinctly forked. 



Although the seven fins above mentioned differ considerably 

 in general form, some being fanlike, while others form fringe-like 

 expansions, yet they generally agree in that they consist of bony or 

 cartilaginous rays, between which is a soft membrane. The rays, 

 however, vary much in character, being sometimes developed into 

 very hard and sharp spines, and sometimes quite soft and flexible. 

 The fins also differ in function, as will be seen when we observe the 

 movements of a fish as it swims. It will then be noticed that the 

 caudal fin, which is spread in the vertical plane and moved sharply 

 from side to side by the powerful muscles of the tail, is the chief 

 propelling organ, while the others are concerned principally in 

 maintaining the balance of the body. This latter point becomes 

 much more evident when we observe the movements of a fish in 

 which one or more of the fins have been injured or lost, as we shall 

 see presently. 



It is true that the pectoral fins are sometimes used to propel, 

 but forward movement is brought about almost entirely by the 

 caudal fin, which acts much in the same way as the blade of the 

 propelling ' screw * of a steam-vessel, the pectorals being used at 



