234 



FISHES 



Median Fins and Appendicular Skeleton 



The Median Fins. Whether existing in the form of a 

 continuous fin, or as discontinuous isolated fins, the median 

 fins are provided with skeletal supports, and also with muscles, 

 primitively formed from intrusive clusters of cells derived 

 from a variable number of the neighbouring myotomes, for 

 their varied movements. The skeletal structures of the dorsal 

 and anal fins consist of a series of bony or cartilaginous, rod- 

 like, and typically tri-segmented radial elements or pterygio- 

 phores, 1 supporting distally a series of dermal structures in 

 the shape of numerous slender horny fibres or ceratotrichia, 

 as in the Elasmobranchii and Holocephali, or a smaller number 



of bony dermal fin- 

 rays, which are prob- 

 ably modified scales 

 or lepidotrichia, 2 as in 

 the Teleostomi. The 

 typical tri- segmented 

 character of the radi- 

 alia is often retained 

 in many existing Elas- 

 mobranchs (Fig. 135) 

 and in Pleuracan- 

 tkus, in Neoceratodus 

 amongst the Dipnoi, 

 in the Chondrostei, 

 in existing Holostei 



FIG. 135. The cartilaginous radialia of the first dorsal f~Ri CT 19C\ nr irl rn 

 fin of Mustelus antarcticus. (From Mivart.) 



greater or less extent 



in several families of Teleosts (e.g. Salmonidae, Esocidae, 

 Cyprinidae, and some Acanthopterygii) ; but in the latter 

 group the radialia are greatly prone to reduction, and hence 

 they are more generally bi-segmented, and sometimes consist 

 of a single proximal segment only (e.g. Gymnotus). In 

 all these Fishes the proximal segments are the longest 

 and the most persistent, and when reduction occurs it is at 



1 Thacker, Trans. Connecticut Acad. iii. 1877, p. 281 ; Mivart, Trans. Zool. 

 Soc. x. 1879, p. 439 ; Bridge, Linn. Soc. Journ. Zool. xxv. 1896, p. 530. 



2 Goodrich, Quart. Journ. Micr. Sci. 47, 1903-1904, p. 465. 



