224 



VERTEBRATE SKELETON 



basalia in Elasmobranchs are included in the body, the radiaha 

 being largely in the fin. In Teleosts the axinosts are wholly within 

 the trunk, the baseosts reaching the surface. At either end of the 

 fin the axinosts alternate (sometimes articulate) with the spinous 

 processes of the vertebrcC, while the baseosts usually articulate with 

 two adjacent axinosts. 



In Elasmobranchs there is no close connexion of actinotrichia 

 with radials, their basal ends lying in the tissue on either side of 

 the radiaha. In Teleostomes, where the ceratotrichia of Elasmo- 

 branchs are replaced by lepidotrichia (p. 221), the skeleton of the 



Fig. 235. — Skeleton of dorsal fin of Pleiironecles (Cole and Johnstone, 'oi). A, 

 just behind head; B, section, a, actinost; b, baseost;/, fin rays; /, ligament; m, muscle; 

 s, base of skull; v, vertebrae. 



free fin (arising as paired structures which fuse) embraces the baseosts 

 (fig. 235, B). The • ceratotrichia are not divided transversely. 

 The lepidotrichia are more or less ossified and present two conditions. 

 Where fully ossified (almost always at the anterior end of dorsal and 

 anal fins) they are strong unsegmented spines (spinous rays of 

 ichthyology). With less lime they are 'soft rays,' usually divided 

 transversely into small articles. Distally the soft rays may branch 

 several times. 



The median fins of IVIalacopterygii, most Physostomes, -Plectognaths and 

 Anacanthini are supported wholly by soft rays. Acanthopterygii have both 

 spinous and soft rays in dorsal and anal tins. 



