508 THE OCEAN WORLD. 



deposited by the milt of the males. Such is a very brief summary of the 

 organization of fishes, which have been briefly described as vertebrate, 

 cold-blooded animals, breathing by means of gills ; living in water, 

 moving through the water by means of their fins, and reproducing 

 their kind by means of eggs, or spawn. And now a few words on 

 their classification. 



Fishes naturally divide themselves into two series, according to the 

 composition of their internal skeleton. This is usually osseous ; never- 

 theless, a whole group of them constantly retain the cartilaginous or 

 fibro-cartilaginous state. With some this frame presents even less 

 power of resistance, and remains membranous. 



It is precisely upon this peculiarity of structure that we found the 

 great division of the class of fishes into two great groups of CARTILA- 

 GINOUS and OSSEOUS fishes, the first being again subdivided into three 

 orders : namely, I. Cyclostomata, II. JSelachia, III. Sturiona. The 

 second into four orders: I. Plectognatliii, II. Lophobranchii, III. 

 Malacopterygii, IV. Acantliopterygii. Agassiz's system of classifica- 

 tion of fish, founded on the form of the scales, is perhaps better suited 

 than this to the palaeontologist, but the one given above, founded as it 

 is principally on the internal parts of the animal, is better suited to 

 the zoologist. Agassiz's orders are the Ctenoid, type, Chromis, 

 Placoid, Ganoid, and Cycloid. 



CARTILAGINOUS FISHES. 



Cartilaginous fishes are generally animals of considerable size, their 

 structure ranging from ordinary fishes to eels. They are chiefly sea- 

 fishes, only a few species being river-fishes. Naturalists divide them 

 into two orders ; namely, those having the gills free on the outer 

 edge (the gilled Chondropterygeans), and those having these organs 

 fixed on both edges. The first order comprehend three families : I. 

 Cyclostomata, or Eels, Lampreys, &c., in which the mouth forms a 

 sucker; II. Selachians, including Kaias and Sharks, in which the 

 mouth is furnished with jaws ; III. Sturiona, or Sturgeons, which 

 have the gills free. 



I. CYCLOSTOMATA. 



The first are characterised by the singular conformation of the 

 mouth, which is formed for suction. The body is elongated, naked, 



