INTllODUCTION. XXI 



summer I was preparing a bottle of fislies for your use, I 

 pierced a Lesser Forked Beard with a pointed probe through 

 the mouth into the air-bladder in order to render the fish 

 small enough to enter the bottle ; but being obliged also to 

 squeeze it with some force for that purpose, the dorsal fin 

 became distended with air — a circumstance that would direct 

 our attention to tlie air-bladder as the source of the air dis- 

 tending the fins and tunic of the eyes in the Gadidae.'''' 



The analogy to the air-cells in birds, and the passage of 

 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 arc no 

 very obvious external signs by which the sexes in fishes can 

 be distinguished. As in the liigher 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 com- 

 pared 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 ex- 

 clusion to shift for themselves. Of these some notice is 

 taken in the body of the work Avhen describing the particular 

 species. The sexual parts are of a higher degree of organiza- 

 tion 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 described 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 



