STRUCTT'KE OF THE FINS. 5 



the anal fin, which is in the inferior median line of the body. And, finally, 

 the fish terminates posteriorly in the caudal fin. These fins vary greatly in 

 size, composition of their skeletons, and form, being sometimes rounded, 

 sometimes pointed, sometimes trnncate. In the embryo fish these vertical fins 

 are not differentiated from each other, but extend as a continuous fringe rouiid 

 the hinder part of the body, and in a few fresh-water fishes, like eels, this 

 condition is retained throughout life. 



The rays which form the finsai'e of three types : first, simjde bony spines ; 

 secondly, softer spines, which are jointed in the upper part; thirdly, soft rays, 

 which are not only jointed in the upper part, but sub-divide so as to terminate 

 in a fringe. The rays which project above the body are jointed at their bases 

 and articulated to internal portions of the fins, which extend between the ex- 

 tremities of the spinous processes of the vertebrae, especially on the dorsal side. 

 In the Acanthopterygian group the first dorsal fin consists of spiny rays, and 

 the second dorsal usually consists of soft- jointed rays, with one or two more or 

 less developed spines in front. The number of rays in a fin is counted, and 

 written in a formula. Thus, in the Perch, the formula of the dorsal fins is 

 written 1 J). 13 — ^15, 2 D. 1 — 2/13 — 14- : signifying that there may be thirteen 

 to fifteen spines in the first dorsal fin, and in the second dorsal Hn, one or 

 two spines followed by thirteen to fourteen soft rays. The other fins have 

 their rays counted and expressed in a similar way, so that, as the anal 

 fin has two spinous rays and eight or nine soft rays in the Perch, its 

 formula is written — A. 2/8 — 9. In this fish the ventral fin has one spiny 

 ray and five soft rays, so that its formula is V. 1/5. The formula of the 

 pectoral fin is P. 14, and of the caudal fin C. 17. In each genus the species 

 commonly vary in the number of rays in some or all of the fins, but the most 

 variable are the dorsal and anil fins. In describing a fish, it is necessary to 

 state the form, positions, and frequently the length of the fins, when the length 

 is in any way unusual. In the Salmon tribe there is a second dorsal fin which 

 contains no rays, but consisting of fatty substances, is known as the adipose 

 fin. 



When a fish is deprived of its fins it floats with the abdomen upward. 

 They therefore maintain the animal in its vertical position, and are organs of 

 motion. 



Scales.' 



There are two p.rincipal types of scales among fresh-water fishes : first, 

 the ctenoid scale, in which the free margin of the scale, and sometimes the 

 whole exposed surface, is serrated or spiny; and, secondly, the cijcloid scale, in 

 which the margin shows no serrations, though it may be marked with delicate 

 rays (Fig. 1). Ctenoid scales are well seen in the Perch and Po])e, while the 



