222 



INTEOBUCTION TO ZOOLOGY. 



structure of these organs 



-^^^ 



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 yoimg 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 fui'ther 

 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 



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 

 {Firi. 193), or in that 

 of the entire fish {Fig. 

 Fig. 193.— DoKSAL Fin. 181) ; while in the 



Trout the corresponding fin is formed of soft flexible rays, 

 dividing into bi'anches. A difference of the same kind is ob- 

 servable 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 difference 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 {Acanthop- 

 tetygii), and those with the fins entirely of soft rays {Malacop- 

 terygii).* These orders are again subdivided, according to 

 the presence or absence of certain fins — the diflTerence in their 

 relative positions — the variety in the structure of the gills and 

 gill-covers, and other details of secondary importance. By 

 these characteristic distinctions the ichthyologist, or in other 

 words the naturalist who makes fishes his peculiar study, ar- 

 ranges them in groups, distinguished as orders, families, and 

 genera. 



* Tliese 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. 



