282 ELEMENTARY ZOOLOGY. 



The fins are, typically : a median fin along the back (dorsal) 

 a large median fin over the tail (caudal), and one ventra' 

 just in front of the tail (anal); a pair of fins on the side 

 close to the gills (pectorals); and a pair, variable in posi- 

 tion, but usually further back and closer to the bell} 

 (pelvics) . Of these the powerful tail fin is the one mosl 

 used in locomotion; the others probably aid in steering 

 and in holding an upright position. The pectoral and 

 pelvic fins are believed to correspond to the arms anc 

 legs of higher animals, but they never have the fingen 

 and toes which are so characteristic of the higher types 

 Examine all the figures that you can find showing varia- 

 tion in the bony structure of fins of fish. The most oJ 

 the fishes produce some sort of scales in the skin. These 

 are bony or horny plates that overlap like shingles and 

 make a protective armor. In some forms, as in the cat- 

 fish and eels, they are wanting or are so deep set in the 

 skin as to be invisible. In others, as the sturgeon and 

 gar, the scales may be fused in such a way as to make a 

 rather heavy armor. 



295. The Skeleton and Muscles; Locomotion. The 



internal skeleton of the fish supports the muscles, and 

 these two elements give the form to the body. In its 

 simplest form the skeleton is cartilaginous, as in the 

 sturgeon and some sharks, and in the embryos of all 

 fishes. It consists of a case in which the brain lies 

 (cranium), followed up by a series (twenty-four, more 01 

 less) of vertebrae extending along the back clear to the 

 tip of the tail. This is the axial skeleton. To the cra- 

 nium a great many bones, loosely fused, are attached 

 (Fig. 1 08). These form the jaws and gill-arches. From 

 the vertebrae of the body arise ribs which partly sur- 



