646 PISCES. 



cases to be nearly pure nitrogen ; but in fishes that live at a great depth, 

 Messrs. Configliacchi* and Biot ascertained that oxygen was substituted, 

 whence it has been presumed that this apparatus was in some way or 

 other an auxiliary in respiration ; and some authors have even gone so 

 far as to see in the swimming-bladder the representative of the lungs 

 of aerial Yertebrata. But, however this may be, the gas enclosed is 

 indubitably a product of secretion, being derived either from the lining 

 membrane of the viscus, or from a glandular structure which may fre- 

 quently be distinctly pointed out in its interior. ^ 



(1763.) Cuvier justly observes that, whatever opinions may be enter- 

 tained relative to the use of the air-bladder, it is difficult to explain 

 how so considerable an organ has been refused to so many fishes not 

 only to those which ordinarily remain quiet at the bottom of the water, 

 as Skates and Plat-fishes, but to many others that apparently yield to 

 none either in the rapidity or facility of their movements, such as the 

 Mackerel for instance ; yet even while the common Mackerel (Scomber 

 scomber) has no air-bladder, a very nearly allied species (Scomber pneu- 

 matophorus) is provided with one ; and of this many other instances 

 might be adduced. 



(1764.) From the circumstances under which fishes seize and swallow 

 their prey, it must be evident that they are incapable of enjoying any 

 very refined sense of taste. Those species which are carnivorous are of 

 necessity compelled to catch with their teeth, and thus retain a firm hold 

 of the active and slippery food they are destined to devour. To divide or 

 masticate their aliment would be impracticable ; and even were they per- 

 mitted so to do, the water which perpetually washes through the interior 

 of their mouths would obviously preclude the possibility of appreciating 

 savours. In the construction of the mouth of a fish we therefore find, 

 generally speaking, that every part has been made subservient to pre- 

 hension : teeth, sometimes in the form of delicate spines, or else present- 

 ing the appearance of sharp recurved hooks, have been fixed in every 

 possible situation where they could be made available as prehensile 

 organs : not only are the jaws densely studded with these penetrating- 

 points, but they are occasionally placed on every bone which surrounds 

 the oral cavity or supports the entrance of the pharynx. The inter- 

 maxillary, the maxillary, and the palatine bones, the vomer, the bran- 

 chial arches, the pharyngeal bones, and even the tongue itself, may all 

 support a dental apparatus, either of the same description or composed 

 of teeth of different shapes ; generally, however, some of these bones 

 are unarmed, and occasionally teeth of any kind are altogether wanting. 



(1765.) But if such is the most usual arrangement of the dental ap- 

 paratus in fishes, we must be prepared to find, in a class so extensive 

 as that we are now investigating, various modifications both in the form 



* " Sull' analisi dell' aria contenuta nella vescica natatoria dei Pesci." Pavia, 

 1809, 4to. 



