368 ZOOLOGY. 



has a protective value, as the discharge is in some cases power- 

 ful enough to paralyze much larger animals than the fish itself. 

 It is probably useful also in capturing prey. 



Asymmetry. In the flat-fishes we find a very striking com- 

 pression from side to side. In early life they have the posi- 

 tion normal to other fish, but in the adult stage they rest and 

 swim with the dorso- ventral plane horizontal instead of verti- 

 cal on the left side in some species and on the right in others. 

 The side that is uppermost becomes pigmented like the back, 

 and the under side loses its pigment and becomes white, as the 

 belly of fishes in the normal position. The eye which belongs 

 to the under side changes its position until it comes to lie on 

 the upper side. The bones of the cranium, especially those 

 about the eye, are twisted and the right and left branches of 

 the jaw are unequally developed. The dorsal and ventral fins 

 become continuous and the body tends to become bilaterally 

 symmetrical in the new position. We can scarcely doubt that 

 this asymmetrical condition has been brought about by the 

 position which the animal takes in relation to the environment, 

 but we know that in some species the eye begins to migrate 

 now before the fish comes to lie on its side. 



385. Classification of Fishes. 



Subclass I. Elasmobranchii (Sharks, Dog-fishes, Rays, Skates). Marine 

 fishes with essentially cartilaginous skeleton ; no operculum or gill-cover ; 

 mouth on the ventral surface of the head ; heterocercal tail ; external 

 skeleton of placoid scales ; spiral valve in the' intestine ; no air bladder. 

 The elasmobranchs are regarded by some as being the nearest present 

 relatives of the primitive fishes. They occur most abundantly and 

 are larger individually in warm seas. They are powerful swimmers as 

 befits carnivorous, preying animals. They feed on Crustacea, mollusca, and 

 fish. 



Subclass II. Ganvidei (Ganoid Fishes: Sturgeon, Gar-pike). Fishes 

 with bony or cartilaginous skeleton ; gills covered by an operculum ; exo- 

 skeleton of ganoid scales or enameled plates; air-bladder present; spiral 

 intestinal valve ; tail either homo- or hetero-cercal. 



The group was very important in the early history of the earth, and 

 the present species are a mere remnant of the former glory of the ganoids. 

 They occur now chiefly in the rivers and lakes, though the sturgeon is also 

 found in the sea. North America is as well represented as any other 

 region in the living species of this remarkable group. 



