APPENDICULAR SKELETON 



225 



The caudal fins (diphycercal excepted) differ from dorsal and 

 anal in the apparent absence of pterygophores, the actinotrichia 

 being supported on the spinous processes of the vertebras. Both 

 arches, neural and hasmal are about equally developed in the diphy- 

 cercal tail, diminishing in size towards the tip, the actinotrichia 



Fig. 236. — developing caudal skeleton of Salmo (Schmalhausen, '12). .4, 16 mm. 

 larva; B, 19 m.m. larva, a. actinotrichia; ch, notochord; d, n, neural arches; h, hasmal 

 arches; hy, hypurals; s, spinous process. 



being supported on elements radial in character. The upward bend 

 of the vertebral column in heterocercal caudals results in the reduc- 

 tion of the neural arches and hypertrophy of the haemals. The same 

 conditions persist in the (homocercal) Teleosts the haemals being 

 greatly expanded) (figs. 236, 237), forming h)rpural bones, often fused 



Fig. 237. — Skeleton of caudal fin of Pleuronecles (Cole and Johnstone 'oi). A, 

 17 mm. larva; B, adult; e, epurals; h, hypurals; hs, hasmal spines; n, neural spines; nc, 

 notochord; u + h, fused 3d hypural and urostyle. 



to large plates. In Teleosts no separate centra form in the tip of 

 the caudal fin, but a rod-like sheath of bone, the urostyle, more or 

 less completely surrounds the end of the notochord. Eels have the 

 hypurals reduced, the urostyle short and the lobe of the fin extended 



