358 



ZOOLOGY. 



ventral, near the anus. These may represent remnants of a 

 continuous median fin such as is seen inAmphioxus (Fig. 152). 

 They are supported by fin-rays in the dermal fold, which 

 are in turn supported by spines imbedded in the muscles. The 

 form of the caudal fin, which is much used in locomotion, 

 differs widely. These differences, correlated with modifica- 



FIG. 176. 



FIG. 176. Diagrams of some principal forms of tails in fishes. A, protocercal fin 

 (as in Polypterus) ; B, heterocercal (as in Sharks) ; C, homocercal (as in most Teleosts) ; 

 D, homocercal (as in Atnia). By Folsom. 



Questions on the figure. What is the essential difference between the 

 symmetry of D and of A? What conceivable advantage has the sym- 

 metrical over the unsymmetrical type? Are the heterocercal types success- 

 ful swimmers? 



tions of the end of the vertebral column, have considerable 

 importance in subdividing the class. The following types 

 may be noted: 



1. The vertebral column passes straight to the end of the 

 tail and the fin-rays are disposed symmetrically with regard 

 thereto .(protocercal) ; found in lung-fishes and some primitive 

 extinct forms (Fig. 176, A). 



2. The vertebral column is bent dorsad, and a small fin lobe 

 develops from its ventral side. The tail, though two-pronged, 

 is not symmetrical (heterocercal). Found in sharks and many 

 ganoids (Fig. 176, B). 



