THE STRUCTURE OF FISHES 



53 



no neck, l)ut the end of the gill cover, or opercle, marks the posterior 

 boundary of the head. It is usually difficult to determine where the 

 trunk ends and the tail begins. The tail is usually defined as the region 

 posterior to the vent. The tail and the tail fin are the important organs 

 of locomotion. The other fins are used by most fishes for balancing, 

 turning, and stopping. 



Diagram 1. Hypothetical drawing of a spiny-rayed fish, (a) Lower jaw, or 

 mandible, (b) Premaxillary. (c) Maxillary, (d, e) Preof)ercle. (f) Interopercle. 

 (g) Opercle. (h) Subopercle. (i) Ventral, or pelvic, fin. (j) Pectoral fin. (k) 

 Anus (vent) . (1) Anal fin. (m) Caudal peduncle, (n) Depth of soft caudal 

 peduncle, (o) Caudal fin. (p) Lateral line, (q) Soft-rayed portion of dorsal fin. 

 (r) Spinous portion of dorsal fin. (s) Insertion of dorsal fin. (t) Nape (occiput) . 

 (u) Cheek, (v) Eye. (w) Nostril (nares) . (x) Snout, (y) Length of head. 



The shape of the body varies, but the most characteristic shape is a 

 streamlined modification of an elliptical or ovoid form. Some species, 

 like the sunfishes, are thin and deep-bodied (compressed) . Others, such 

 as the catfishes, are broad and shallow-bodied (depressed) . The most 

 extreme body shape found among fresh-water fishes is the long slender 

 form of the eel. The length of a fish is usually considered by fish spe- 

 cialists to be the length from the tip of the nose to the end of the 

 vertebral column at the base of the tail fin. This measurement is known 

 as standard length and is usually the length referred to in technical 

 descriptions and keys for identification. 



Since the size of the mature fish is indefinite, most of the measure- 

 ments used are comparative, and even these vary for different age 

 groups. Head in length means the number of times that the distance 

 from the tip of the snout to the posterior edge of the opercle including 

 the membrane is contained in the length of the fish. Depth in length 

 means the number of times that the greatest distance from the back, 



