222 



VERTEBRATE SKELETON 



Teleosts. The dorsal fin is frequently divided into two or more 

 parts, while the anal is rarely interrupted, though sometimes absent. 

 The caudal fin undergoes the most modifications (fig. 233). In 

 what must be regarded as the primitive condition, the vertebral axis 

 extends in a straight line through the tail, and the caudal fin is 

 arranged symmetrically about it (diphycercal tail). This is the 

 earhest stage in all fishes and it persists in Cyclostomes. Dipnoi and 

 some Teleosts. In all others this stage is followed by a heterocercal 

 tail in which the vertebral axis is bent upwards, while the anterior 

 part of the ventral fold (hypochordal lobe) is greatly enlarged, so that 

 the whole fin appears as if composed of two unequal lobes, the verte- 

 bral axis extending into the upper of these. Aniia (Ganoid) and 



Fig. 233. — Tails of fishes. A, young Amia; B, diphycercal; C, heterocercal; D 

 homocercal; h, hypurals; n, notochord; s, spinal cord. 



Teleosts pass through this heterocercal condition which is permanent 

 in other Ganoids and in Elasmobranchs, and then have a greater 

 development of the hypochordal lobe, the result being that the 

 upper and lower lobes appear equal (homocercal) although inter- 

 nally they retain the bent vertebral a.xis. The homocercal tail 

 may be rounded or excavate at the end. 



As implied above, Amia agrees with most Teleosts in being homocercal. 

 Polypterus, many eels and a few other Teleosts are diphycercal, probably a 

 retrograde condition. 



Caudal fins are developed in a few other \'ertebrates. Ichthyosaurs have a 

 heterocercal tail, but here the vertebral column extends into the ventral lobe of 

 the tail. In Cetacea and Sirenia the tail bears horizontal rather than vertical 



