INTERNAL ANATOMY OF FISHES. 13 



only. In no fishes, however, is there any medullary 

 canal. The more perfect groups or orders have ribs, 

 but these disappear in many of the fibro- cartilaginous 

 genera ; and finally, in such as pass into the annulose 

 animals or insects, the whole skeleton is soft and mem- 

 branaceous : first the fins, then the eyes, and, finally, 

 the vertebrated column itself, almost disappear, so that 

 we have the mere external form of a worm, provided 

 with a mouth. 



(14.) As fish are destined to inhabit an element 

 where motion is much more essential to them than 

 either to quadrupeds or birds, their Omnipotent Creator 

 has given them greater powers for sustaining this motion 

 than are possessed by any other animals in creation. 

 Their body, in fact, is surrounded by fins ; and their 

 tail (the fin of which acts as a rudder) is generally 

 as thick, and often much longer than the body itself. 

 These are the only members adapted for motion pos- 

 sessed by fishes ; but their construction, number, and 

 position, are varied in almost an infinity of ways, 

 and thus contribute some of the most obvious and na- 

 tural characters for determining the different families 

 and genera. As the formation of the fins comes under 

 the head of external anatomy, we shall subsequently 

 treat of these members more at large. 



(15.) The air-bladder is situated immediately under 

 the spine : by being compressed or dilated, it influences 

 the specific gravity of the fish, and assists it in rising 

 or descending in the water. This vessel, however, is 

 very partially possessed ; and even its presence or ab- 

 sence may be detected in genera, and even species, 

 which are closely and internally allied, so that it be- 

 comes of no value whatever as a character for desig- 

 nating groups. 



(16.) The mouth is sometimes provided with very 

 strong teeth, and sometimes entirely without ; and this 

 remarkable variation takes place in genera close to each 

 other, and even, according to some of Cuvier's groups, 

 in species of the same sub-genus. The anatomical 



