APPENDICULAR SKELETON 



221 



Actinotrichia differ in Elasmobranchs and other fishes. In the former they 

 are horny and non-cellular, and resemble elastic-tissue fibres (ceratotrichia). 

 In Teleosts, Ganoids and Dipnoi they contain bone cells and are called lepido- 

 trichia, and may, to varying extents, be converted into bone, or may be enclosed 

 in boLe of dermal origin. 



MEDIAN (Azygos) APPENDAGES 



Although median appendages appear here and there in higher 

 classes (aquatic Urodeles, larval Anura, Crocodiha, dorsal crests of 

 some lizards and dorsal fins of Ichthyosaurs and whales), they have 

 an internal skeleton only in Cyclostomes and Fishes, apart from 

 the spinous processes of the vertebrae. Hence all except these two 

 divisions may be dismissed without further mention. Before 

 describing the skeleton of the median fins, a word is necessary 

 concerning their modifications. 



The median fins (pinnae) are primitively derived from a con- 

 tinuous median fold, beginning on the back, just behind the head, 



Fig. 232. — Hexayichus griseus (Jordan and Everm^nn, '00). a, anal; c, caudal; 

 d, dorsal; p, pectoral and v. ventral fins, an and po, anterior and posterior borders ot 

 fin; /, nearly horizontal line of attachment to trunk; the side of the pectoral fin shown is 

 dorsal side. 



extending around the tip of the tail and passing forwards on the 

 ventral surface as far as the anus. The posterior part of this fold 

 (caudal fin) is most strongly developed and is usually the principal 

 locomotor organ, the other parts of the fold acting more as keels. 

 From this continuous fold all median fins are derived by overgrowth 

 (hypertrophy) of parts and obsolescence or complete disappearance 

 of others. With this formation of gaps one or more dorsal fins 

 (fig. 232) are differentiated from the caudal fin, which, in turn, is 

 similarly separated from an anal fin, on the ventral side just behind 

 the anus. The caudal excepted, any of these fins may disappear, 

 and the caudal may be greatly reduced as in skates and some 



