INTRODUCTION. XX111 



air from thence into the bones of the limbs, is too obvious to 

 be unobserved, and will give interest to further investiga- 

 tion. 



Except in the Cartilaginous Sharks and Rays, there are no 

 very obvious external signs by which the sexes in fishes can 

 be distinguished. As in the higher animals, however, the 

 respiratory organs occupy more space in the males than in the 

 females ; and on the other hand, the abdomen is larger in the 

 females than in the males : the males may therefore be known 

 from the females by their somewhat sharper or more pointed 

 head, the greater length of the gill-cover, and the body from 

 the dorsal fin downwards being not so deep compared with 

 the whole length of the fish. 



Among fishes generally a few are viviparous, bringing forth 

 their young alive, which are able from the time of exclusion 

 to shift for themselves. Of these some notice is taken in the 

 body of the work when describing the particular species. 

 The sexual parts are of a higher degree of organization in the 

 Sharks and Rays, and more complicated in their structure 

 than those of the bony fishes, resembling the sexual organs in 

 reptiles ; and their mode of producing their young is describ- 

 ed also at the commencement of the history of each, and need 

 not therefore be repeated here. 



The sexual organs in by far the greater number of fishes 

 are much more simple, consisting, as will be found towards 

 the season of producing their young, of two elongated oval 

 lobes of roe, one on each side of the body, placed between 

 the ribs and the intestinal canal ; these lobes, in the female 

 called hard roe, contain a very large number of roundish 

 grains called ova or eggs, which are enclosed in a delicate 

 membranous tunic or bag, reaching to the side of the anal 

 aperture, where an elongated fissure permits egress at the 

 proper time. In the males, the lobes of roe are smaller than 

 in the females, and have the appearance of two elongated 



