76 VERTEBRATA. 



The first peculiarity that strikes an observer on 

 comparing a Fish with almost any other vertebrated 

 animal, is, the simplicity of its figure, the absence of 

 those prominent limbs which form so large a part of 

 others ; the head, without any visible neck, merges 

 into a more or less rounded tapering body, which 

 terminates in a broad tail, in an almost unbroken 

 outline, for the fins are generally so slight and mem- 

 branous in their texture as scarcely to diminish this 

 unity of form. Placed in a fluid of very nearly 

 the same weight as itself, the Fish needed not the 

 stout legs of the Quadruped, nor the long wings of 

 the Bird. In order to rise, or descend at pleasure, 

 most Fishes are provided with a long air-bladder in 

 the upper part of the belly, by the compression or 

 relaxation of which the body is made heavier or 

 lighter than the surrounding water. The great organ 

 of progression is the tail, which is thick and muscu- 

 lar, and being dilated by a broad finny expansion in 

 a vertical direction, strikes the water to the right 

 and left alternately with great power, as a single oar 

 is used in the common operation of sculling a boat. 

 The use of this organ as a rudder to change the 

 direction is of course obvious. The pectoral, or 

 breast fins, and the ventral or belly fins, likewise 

 contribute to progressive motion, when the impulse 

 is intended to be slight and short; but their chief 

 use seems to be the balancing of the body : although 

 it is certain that they represent, the former, the 

 fore-limbs, and the latter (notwithstanding their 

 position) the hind limbs of higher animals. The 



