FISHES 



19 



fins XI-III, 5" means that the anterior dorsal fin has 11 spiny rays 

 and the posterior dorsal has 3 spiny and 5 soft rays. In certain groups 

 of soft-rayed fishes, such as the trout, catfish and others, the posterior 

 dorsal fin is a fleshy structure without rays of any kind, and is conse- 

 quently not mentioned in such descriptions; it is called the adipose fin. 

 Lateral Line (Fig. i). — In most fishes the lateral line appears as a 

 longitudinal series of modified scales extending along the side of the 



body from the head to the caudal fin. 

 In the lower, soft-rayed fishes it tends 

 to run parallel to the line of the belly, 

 and in the higher, spiny-rayed fishes to 

 the line of the back. In many fishes it 

 is not complete or is altogether wanting, 

 at least so far as any external appear- 

 ance is concerned. 



Scales (Fig. 5). — In some species of 

 fishes, the catfish for example, the skin 

 is naked and without any hard skeletal 

 covering. In most fishes, however, the 

 skin is protected by a layer of scales, and 



Fig. 3. — Position of the ventral fins: 

 a, the ventral fins in an abdominal 

 position; h, in a thoracic position; c, in 

 a jugular position: i, pectoral fins; 2, 

 ventral fins {from Siissw. Fauna Deut.). 



Fig. 4. — Soft fin-rays (a); spiny fin- rays (6) 

 {from Siissw. Fauna Deut.). 



in a few groups by bony plates, prickles, or other skeletal structures. By 

 far the greatest number of species have scales of one of two types, the 

 cycloid or the ctenoid scales. These are thin rounded plates which cover 

 the body more or less completely and are imbricated, that is, arranged 

 in overlapping rows like the shingles on a roof, the free ends being 

 directed backwards. Cycloid scales characterize the more primitive 

 of the bony fishes, as the trout and salmon, and have a smooth margin. 

 Ctenoid scales have a more or less roughened surface and a dentate, 

 serrate or spiny free margin; they characterize the more specialized 



