i 



FINS 63 



undergone in certain fishes in order to fit them for the per- 

 formance of new functions for which they were not originally 

 intended. It will be convenient to deal first with the median 

 fins, commencing with the dorsal. 



In the Sharks {Pleurotremata) the dorsal fins retain their 

 primitive function of acting as stabiHsing keels, but in the Rays 

 (Hypotremata), fishes adapted for a life on the sea floor, the need 

 for such keels has disappeared and the dorsal fins are pro- 

 gressively reduced (Fig. 8a), until in the more speciaHsed forms 

 such as the Sting Rays (Trygonidae) and Eagle Rays {Mylio- 

 batidae) they are altogether wanting (Figs. 8a; 14B; 32B). In 

 some of the Sharks, notably in the Bull-headed Sharks {Hetero- 

 dontidae) of the Pacific and the so-called SquaUd Sharks 

 {Squalidae) , each of the two dorsal fins is preceded by a^ stout, 

 sharp spine (Figs. 53B; 6oa). The origin of these spines is 

 somewhat obscure, but they are believed to owe their existence 

 to the fusion of some of the shagreen denticles covering the 

 front parts of the fins {cf. p. 85). Such spines provide for- 

 midable defensive weapons, especially when associated with 

 poison glands, as in our own Spiny Dog-fish [Squalus) [cf. p. 140). 



Many of the Sharks now extinct possessed similar spines, and, 

 not infrequently, where the remainder of the fish's body has 

 been destroyed, these spines, which are grouped together 

 under the name of " Ichthyodorulites," are the only record left. 

 Many have been discovered in Devonian and Carboniferous 

 strata, some saw-edged, some smooth, some straight, some 

 curved, and some with elaborate sculpturing. The owners of 

 some of these spines may never be discovered, but must have 

 been of gigantic build, for a fin-spine found in the carboniferous 

 limestone of Bristol measured no less than three feet in length. 



Among the Bony Fishes the dorsal fin exhibits great diversity 

 both in size and form, and sometimes becomes specially modi- 

 fied for the performance of peculiar functions. It is rarely 

 wanting altogether, but in the group of South American fresh- 

 water fishes known as Gymnotids, to which the Electric Eel 

 (Electrophorus) belongs, it is either absent or reduced to a mere 

 fleshy filament (Fig. 6og). In the more primitive forms the 

 fin (or fins) is supported entirely by flexible and articulated 

 rays, those at the front end generally being simple, while the 

 majority are branched at their tips. Such fishes were grouped 

 together by the older naturaHsts as Malacopterygians (soft fins) 

 to distinguish them from the Acanthopterygians (spiny fins), in 

 which the rays supporting the front parts of the dorsal and anal 



