3*1 



FISHES. 



8— 



elasmobranchs there are no membrane-bones (as the elements of the skeleton not 

 formed from primitive cartilage are termed), in the higher bony fishes the pectoral 

 girdle, as shown in the figure on p. 316, comprises a scapula and a coracoid, 

 flanked by a series of membrane-bones, known as the post-temporal, supra- 

 clavicular, clavicular, and postclavicular. The pelvis 

 is generally absent, and is never highly developed. In 

 all cases the basal and radial bones of the pectoral fins 

 articulate directly with the pectoral girdle, so that there 

 are no segments corresponding to the arm and fore-arm 

 of the higher Vertebrates. In the paired fins the struc- 

 ture is very similar to that of the tail ; and a similar 

 transition from a fringed to a fan -like type may be 

 traced as we pass from the primitive to the specialised 

 forms. For instance, in the figure of the perch's 

 skeleton on p. 316, we may notice that the paired fins 

 J are formed of a number of hard rays spreading out in 

 ] a fan-like manner from a single point of origin ; and 

 \ the same general type obtains in the existing sharks and 

 \ rays. In certain extinct sharks, like the one of which 

 the skeleton is shown on p. 317, as well as in the lung- 

 5 fishes and the fringe -finned ganoids, the pectoral fins 

 : have a long central lobe running for some distance up 

 \ the middle, and completely covered with scales (where 

 ^ these are developed), while the rays of these fins form 

 j a kind of fringe radiating on all sides from the central 

 X lobe. The skeleton of such a fin, which is known as 

 3 an archipterygium, consists of a long cartilaginous axis, 

 > composed of a number of joints, gradually decreasing in 

 $ size from the base to the extremity, as shown in the 

 I figure on p. 319. From one or both sides of such 

 joints there are given off a number of oblique smaller 

 jointed rods, terminating in the fine rays forming the free 

 edges of the fins. How different is the structure of this 

 fin from that of the higher bony fishes will be apparent 

 by comparing the accompanying figure with that of the 

 skeleton of the perch on p. 316. In the lung-fishes this 

 primitive type of fin has persisted to the present day ; in 

 the sharks it has now totally disappeared ; while among 

 the bony fishes and ganoids, in the latter of which it was 

 the universal type at the period of the Old Red Sandstone, 

 it now only remains in a modified form in the bichir of 

 the Nile, having been developed in the modern bony fishes into the fan-type. It 

 may be mentioned that the latter modification of fin is obviously the one best 

 adapted for quick-swimming fishes, the fringe-finned type partaking more of the 

 nature of clumsy paddles, and being adapted for slowly-moving forms like the 

 lung-fishes, which pass most of their time among the mud at the bottom of rivers. 



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