CARTILAGINOUS FISHES. 255 



of temperature, would be softened and ultimately dis- 

 solved by long maceration in water. Those of sea ani- 

 mals are, on the other hand, composed of what is called 

 mother-of-pearl, from the play of colours usually ob- 

 servable in it. It consists of coagulated albumen and 

 carbonate of lime, in very thin layers ; a structure much 

 better fitted for bearing the action of water, than that 

 of the air and variations of temperature. 



The breathing apparatus of those fishes is a curious 

 structure. With the exception of the sturgeon, which 

 has some other peculiarities, the gills in all are fixed, 

 and inclosed in a thorax or chest, furnished with carti- 

 laginous ribs, and a cartilaginous diaphragm, and thus 

 capable of being extended and contracted like that of 

 the mammalia. On this account, the romance writers on 

 natural history have described the cartilaginous fishes 

 as breathing with lungs, and being intermediate between 

 the cetacecB and the osseous fishes ; at the same time 

 that they had gills. Thus furnishing them with two 

 sets of respiratory apparatus, and yet with but one ven- 

 tricle and auricle in the heart. 



Now the fact is, that if we are to consider the animals 

 which most resemble man to be the most perfect, (which, 

 by the way, is a very improper mode of expression, as 

 a lamprey, or even a polypus, which is nothing but a 

 little tube or sac, is just as perfect in its way as a 

 greyhound or a race-horse,) if we are to use that mode 

 of expression, we must consider the cartilaginous fishes 

 as a degree lower than the osseous, on account of their 

 soft skeleton and their mode of respiration, and also 

 in their nervous structure. They correspond in another 

 particular : those animals which are usually accounted 



