158 FISHES 



Although as a rule smaller in size, the pelvic fins bear a 

 general resemblance to the pectoral fins, but in certain groups, 

 especially in Teleosts, they are liable to undergo extraordinary 

 changes in position, and, a,s will be seen presently, are much more 

 prone to exhibit the effects of adaptive modification and degenera- 

 tion. They are present in all existing Fishes, with the exception 

 of the Crossopterygian Calamichthys and some Teleosts, and, 

 except in the Teleostei, they invariably retain their primitive 

 position near the junction of the trunk with the tail, and directly 

 in front of the cloacal or the anal aperture ; in this position 

 they are said to be " abdominal." In other Teleostei the fins 

 undergo forward displacement and come to lie directly beneath 

 the pectorals (Fig. 415), when they are said to be "thoracic," 

 as in the Mackerels (Scombridae) and the Horse -Mackerels 

 (Carangidae) ; or even in front of the pectoral fins on the under 

 surface of the throat, when they are described as "jugular," as 

 in the Cod and other Gadidae (Fig. 398). 



Both the median and the paired fins are supported by an 

 internal skeleton, consisting (i.) of a series of cartilaginous or 

 bony, rod-like radial elements or pterygiophores, for the support 

 of the inner or proximal portion of the fins, and (ii.) of a series 

 of horny fibres, or bony dermal fin-rays, which fulfil a like 

 function for the outer or distal portion. The radial elements, 

 however, are never visible externally, even when, as in most 

 Elasmobranchs, they support the greater part of the fins, 

 inasmuch as they are invested by the fin-muscles and the skin ; 

 and in the same group, where horny fibres complete the fin- 

 skeleton, they too are covered by the spinose skin, and hence 

 offer no external evidence of their existence. In the Teleostomi 

 a marked reduction in the number and length of the radial 

 elements of the paired fins, and the insinking of those pertaining 

 to the median fins into the adjacent muscles of the body-wall, 

 leaves the dermal fin-rays, with their thin covering of transparent 

 and usually scaleless skin, as obvious features in the external appear- 

 ance of the Fish, and apparently as the sole support of the fins. 



The dermal fin-rays of the Teleostomi exhibit an obvious dis- 

 tinction into spines and soft rays (Fig. 93, A). The former are 

 stout, rigid, and unbranched structures, pointed at their free distal 

 ends, which, in numbers differing in different genera and species, 

 support the anterior portions of the dorsal, anal, and pelvic fins. 



