256 CARTILAGINOUS FISHES. 



the least perfect, are the most tenacious of life ; and, 

 generally speaking, cartilaginous fishes are much more 

 so than those that have bones. 



In those fishes the gills consist of a greater or smaller 

 number of bags or cells, the internal surface of which 

 is covered with fleshy fibres, the same in appearance 

 as the gills of osseous fish, and, no doubt, answering 

 precisely the same purpose, that of affording the blood 

 the oxygen necessary for the purposes of life, by ex- 

 posing it to water containing that fluid, in an extended 

 tissue of minute vessels. Those gill cells vary in 

 number in different genera, there being seven on each 

 side in the lamprey, and only six in the hag. The 

 openings which lead from those cells to the surface of 

 the fish, vary even more. The lamprey has seven on 

 each side ; the hag only one, but each opening in that 

 communicates with all the cells. At their other termi- 

 nations, the cells communicate with the gullet, so that 

 they are adapted as a thoroughfare for water, like other 

 gills, and not for alternate respiration and expiration 

 by the same passage, like lungs. 



Analogy would lead us to suppose that the water is 

 received by the mouth, conducted thence by the tubes 

 to the cells, deprived of the oxygen of its air by the 

 fibres in these, and then discharged through lateral 

 openings as useless, by the action of the sides and 

 diaphragm of the thorax. Nor can there be any doubt 

 that when the fish is swimming " free-mouthed," this 

 must be the case. But there are some genera, that 

 use the mouth as a sucker, during which time it cannot, 

 consistently with the principle of suction, have any 

 connexion with the breathing apparatus. Sir Everard 



