FINS 



57 



where the folds joined the body each rod later split into two 

 portions. The lower pieces, known as the basals, were situated 

 within the body, and the upper pieces or radials were in the fins 

 themselves. Something very like this primitive condition is 

 found to-day in the Lampreys and their allies, where the 

 median fins are supported solely by such a series of cartilaginous 

 rods. In the Selachians the radials may be further subdivided 

 into two or more portions, and form the main supports for the fins, 

 sometimes extending almost to their margins (Fig. 24A) . The fins, 

 however, are considerably larger than those of the Lampreys, and 

 further strengthening is provided by the presence of numerous 



•^^ii.sp. 



Fig. 24. — STRUCTURE OF DORSAL AND ANAL FINS. 



A. First dorsal fin of Mackerel Shark {hurm punctatus), dissected to show 

 cartilaginous supports ; B. Anal fin of Chinese Sturgeon (Psephurus gladius), 

 similarly dissected ; c. Skeleton of dorsal and anal fins, and portion of vertebral 



column of Gar Pike {Lepidosteus platystomus). 

 b., basal cartilages ; f.r., fin-rays ; i.sp., interspinous bones ; r., radial cartilages. 



hair-Uke, horny rays situated outside the radials, each ray 

 tapering oflf to a fine point near the edge of the fin. In these 

 fishes the horny fin-rays, as well as the cartilaginous radials, 

 are completely enveloped by the skin and muscles associated 

 with the fins, and are, therefore, quite invisible externally. 



Among the Bony Fishes the Sturgeons [Acipenseridae) , living 

 representatives of a very archaic group of fishes, present fins 

 of a distinctly primitive type. The dorsal and anal fins are each 

 provided with a fleshy lobe at the base, composed of fin-muscles 

 surrounding a series of rod-like structures — basals within 

 the body and radials in the muscular lobe of the fin (Fig. 24B). 

 The whole structure is not unHke that of the Selachians, but 



