TAIL 



553 



and in the earliest bony fishes the vertebral column turns sharply 

 upwards, a condition called heterocercal. The haemal spines 

 and rays which support the lower part of the fin are elongated, 

 so that it is morphologically 

 true to say, as is often done, 

 that the lower lobe of the tail is 

 larger ; this is, however, some- 

 what misleading, as when the 

 tail is looked at from outside, 

 without dissection, the appear- 

 ance may be that of a large 

 upper lobe containing the verte- 

 bral column, with a smaller 

 ventral lobe supported only by 

 spines and rays, as one sees it in 

 the dogfish. In other fish, as in 

 the extinct shark Cladoselache, 

 the two lobes appear of equal 

 size. Heterocercy is found in 

 nearly all elasmobranchs, and in 

 sturgeons. The other type of 

 asymmetrical caudal fin, called 

 hypocercal, or reversed hetero- 

 cercal, is found in the early 

 Agnatha (ostracoderms). It is a 

 mirror image of the heterocercal 

 type ; the vertebral axis turns 

 downward and the neural spines 

 or their radials are elongated. 

 It is found in no living forms. 



The two other types of tail 

 are both externally symmetrical, 

 but show signs, in internal 

 structure, in embryology, in fossil 

 history, or in more than one of 

 these, of a former asymmetry. If 



the symmetry is now complete, both internally and externally, 

 the fin is called diphycercal ; such is the condition in the lamprey 

 and in the hving lung-fishes. The development of the tail of the 

 lamprey shows signs of a hypocercal condition, as would be 

 expected. The last type, the homocercal, has an internal structure 



Fig. 431. — Stages in the develop- 

 ment of the homocercal tail of 

 the flounder {Pleuronectes fiesus). 

 From Goodrich, Studies on the 

 Structure and Developwent of 

 Vertebrates. 1930. Macmillan, 

 London. After Agassiz. 



ac., .Actinotrichia ; c, axial lobe ; ha., lupmal 

 arch ; hf., hypochordal fin ; hy., hypural 

 cartilage ; /., demial ray ; nsp., neural 

 spine ; nt., notochord. 



