dorsal fin 



operculum 



spirocie 



homboid scales 



caudal fin 



ACIPENSER 



scutes P^'^'' fi" 



pectoral fin 



Figure 2-3. External features of two chondrosteans. 



Holosleans and teleosls The Bowfin, Amia, a representa- 

 tive holostean, is a thick-bodied fish with a large head and 

 mouth (Figure 2-4). The head is covered with thick bones 

 but is without scales on the cheek. There is a long dorsal 

 fin, an anal fin, and two pairs of lateral fins. The caudal fin 

 is rounded in outline. The scales of the body are large and 

 cycloid, that is, round in shape and overlapping. The 

 lateral-line system is indicated by rows of pores on the head. 

 The eyes are large and lateral. The anterior nasal opening 

 is at the tip of a nasal tube extending from the upper lip; 

 the posterior opening is high on the snout in front of the eye. 

 The opercular covers are free to a point far forward between 

 the halves (rami) of the lower jaw. These margins end at a 

 median plate (gular plate). There is no spiracle. 



The gar, Lepisosteus, is a very elongated fish with a 

 toothed snout. The nasal capsule with its two openings is at 

 the tip of the snout; the eye lies behind the angle of the 



mouth. The dermal bones of the head are visible through 

 the skin, and the cheek is covered with an intricate and vari- 

 able pattern of small plates. The opercula are joined across 

 the throat. The body is covered with imbricated rhomboid 

 scales with a glassy surface of an enamel-like material 

 (ganoin). The dorsal fin is far back, and there are two pairs 

 of lateral fins and an anal fin. The tail is heterocercal but 

 the fin is not subdivided. 



The perch, Perca, a typical teleost, has cycloid scales 

 covering the body and cheeks. These scales have fine spines 

 along their anterior margin and are thus identified as 

 ctenoid. The dorsal fin has an anterior spined half and a 

 posterior soft lobe. The lateral fins have slight, fleshy, basal 

 lobes and like the other fins are formed of a skin membrane 

 supported by jointed fin rays. The tail is homocercal, that 

 is, it has equal lobes neither of which is supported by an ex- 

 tension of the body. 



dorsal fin 

 spinous 5of, ray 



nasal tube. 



caudal fin' 



pectora 



AMIA 



pelvic fin 

 Figure 2-4. Holostean and teleosteon bony fishes. 



LIVING REPRESENTATIVE VERTEBRATES • 25 



