362 



ZOOLOGY 



anus. These may represent remnants of a continuous median 

 fin such as is seen in Branchiostoma (Fig. 154). They are sup- 

 ported by fin-rays in the dermal fold, which are in turn sup- 

 ported by spines imbedded in the muscles. The form of the 

 caudal fin, which is much used in locomotion, differs widely. 



Fig. 179. 





~-V?~ 



Pig. 179. Diagrams of some principal forms of tails in fishes. A, protocercal fin (as in Poly p- 

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

 Amia). By Folsom. 



Questions on the figures. — What is the essential difference between the sym- 

 metry of D and oi A? What conceivable advantage has the symmetrical over the 

 unsymmetrical type? Are the heterocercal types successful swimmers? 



These differences, correlated with modifications of the end of 

 the vertebral column, have considerable importance in subdivid- 

 ing 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. 179, 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. 179, B). 



3. The vertebral column may become still more bent and 



