FISHES 267 



much shorter than the effective part of the organ. They are described 

 as pectoral and ventral fins, and correspond to the front and hind limbs 

 of other vertebrates. In addition to these paired fins, others are de- 

 veloped along the vertical median line of the body. In their simplest 

 form (see eel) the fins present a continuous fringe or band. In most 

 cases, however, this fringe is broken up by intervening gaps into separate 

 segments, which are known as the dorsal, caudal, and anal fins. (Explain 

 these names, and say in what fishes one or other of these fins is absent. 

 Note the position of the ventral fin.) If the two lobes of the caudal fin 

 are of equal size, the fin is described as symmetrical or homocercal (in 

 most teleostean fishes) ; if the lobes are of unequal size, the fin is said 

 to be asymmetrical or heterocercal (e.g., in sharks and sturgeons). 



SKELETON OF THE COMMOX PERCH. (About one- fourth natural size.) 



Z., Prsemaxilla ; 0., maxilla; U., mandible; Au., orbit; Kd., parts of the operculum ; 

 Br., pectoral fin ; Sch., bones of the shoulder girdle ; Br., ventral fin ; Be., pelvic bones ; 

 W., vertebral column; o.D., neural spines; u.D., haemal spines; R., ribs; G., inter- 

 muscular spinous processes of ribs ; .Ft., interspinous bones ;,R1, first dorsal fin with 

 spinous rays ; R2, second dorsal fin with soft rays ; A., anal fin j S., caudal fin. 



The fins are expansions of skin, supported by bony or cartilaginous 

 rays and connected with muscles, by the action of which they can be 

 expanded or folded together, erected or depressed. If the rays are each 

 composed of a single solid piece, they are known as " spinous rays " or 

 spines ; if, on the other hand, they are transversely jointed and divided 

 longitudinally at their upper portions, they are known as " soft rays." 

 (Examples.) The rays of the dorsal and anal fins are articulated with the 

 so-called " interspinous bones," which are plunged between the muscles 

 of the two lateral halves of the body, and interposed between the dorsal 

 (neural) or ventral (haemal) spines of the vertebrae (see Part I., pp. 3 and 4). 



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