238 



INTRODUCTION TO ZOOLOGY. 



In some fishes, as the Skate and the Shark, the skeleton is 

 cartilaginous, or composed of gristle, being so far analogous to 

 the skeleton of the young of the mammalia before the earthy 

 particles which convert the cartilage into bone have been 

 deposited. In others, as the Perch, the Trout, and the Cod, 

 the skeleton is formed of bone. This points out an obvious 

 division of fishes into two primary groups the cartilaginous 

 and the bony. The latter admit with facility of further 

 division. If we examine the Perch and the Trout, we find 

 the bones of the same material, and the gills formed after the 

 same model. The back in each is surmounted by two fins, 

 but the resemblance ceases when we come to examine the 

 structure of these organs. In the Perch, the first of these 



dorsal fins, or that 

 which is next to the 

 head, is composed of 

 stiff spines united 

 by a membrane, as 

 shown in the annexed 

 figure (Fig. 193), or 

 in that of the entire 

 fish (Fig. 181); 

 while in the Trout 

 the corresponding fin is formed of soft flexible rays, dividing 

 into branches. A difference of the same kind is observable in 

 the anterior or front portion of some of the other fins : the tail 

 fin consists in both cases of the most flexible rays. This dif- 

 ference in the dorsal fin (Latin, dorsum, the back) may seem 

 a very trivial matter; but it enables the naturalist to divide 

 the osseous or bony fishes into two orders those with the 

 fins partly of hard or spiny rays (Acanthopterygii) , and those 

 with the fins entirely of soft rays (Malacopterygii)* These 

 orders are again subdivided, according to the presence or 

 absence of certain fins the difference in their relative posi- 

 tions the variety in the structure of the gills and gill-covers, 

 and other details of secondary importance. By these cha- 

 racteristic distinctions the icthyologist, or in other words the 

 naturalist who makes fishes his peculiar study, arranges them 

 in groups, distinguished as orders, families, and genera. 



Those scientific terms are both derived from Greek words, signifying, 

 in the one case, fins of sharp or spinous rays, and in the other, fins soft or 

 of flexible rays. 



Fig. 193. DORSAL FIN. 



