232 FISHES 



another is possible in a living organism, the propulsion of 

 the fish is effected by vibratory movements of the hinder 

 portion of the body ; the fins are flat, fan-like outgrowths 

 which aid or direct the movement. The tail, or to be 

 more exact, the caudal fin offers the necessary resistance to 

 the water in the propelling motion, and acts like an oar used 

 in the to-and-fro movement of sculling from the stern of a 

 boat ; in the forward part of its movement it is narrowed 

 by the approximation of its rays and turned sideways so as to 

 offer the least resistance to the water, while in the backward 

 movement it offers the greatest surface and the greatest 

 resistance. The caudal fin is vertical, and on the dorsal and 

 ventral edges of the body there are other vertical fins which are 

 median and unpaired, and whose function is to prevent the 

 fish from turning over to one side or the other. The lower part 

 of the body behind the head is enlarged to contain the digestive 

 and reproductive organs and the surface of this abdominal 

 region is broad and rounded and destitute of vertical fins ; in 

 this region, there are, however, two fins on each side, generally 

 horizontal in position, and corresponding to the fore and hind 

 limbs of the backboned animals which walk on land ; the function 

 of these fins is chiefly to preserve the position of the body and 

 to control the direction of the movement ; they are called the 

 pectoral and pelvic fins, the former being close behind the gills, 

 the latter in the more primitive fishes near the anus. The 

 marine animals which most resemble fishes in their organs of 

 locomotion and form of body and which come nearest to them 

 in their mastery of the aquatic medium are the Cetacea or whales, 

 but these are inferior to the fishes by the fact that, being in all 

 essential respects similar to the hairy milk-giving vertebrates 

 or mammals, they are still dependent on the air for respiration 

 and are therefore compelled to rise to the surface at intervals 

 to breathe. In consequence of their apparent resemblance to 

 fishes in form and habits, Cetacea are popularly regarded as fishes 

 but: that they true mammals is proved by the facts that they 

 have traces of hair, that they breath air by lungs, and that they 

 suckle their young. 



In fresh waters many of the marine types are entirely un- 

 represented, and on the other hand some aquatic creatures are 

 found belonging to groups which include no marine species. 



