552 



STRUCTURE OF VERTEBRATES 



THE TAIL 



The general shape of the original vertebrate was presumably 

 that of a fish, and it swam in the water in which it lived chiefly 

 by means of unpaired fins. These were supported by rods of bone 

 or cartilage, and in addition by ' fin rays ' of varied chemical 

 nature. The most important of these unpaired fins is the caudal, 

 which surrounds the tail in a vertical plane ; it has a number of 



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Fig. 430. — Diagrams of tails of fishes. The neural and haemal spines, the radials 

 and dermal fin-rays, which variously support the caudal fin, are not shown. 



A, Hypothetical protocercal type ; B, sturgeon, heterocercal type ; C, herring, homocercal type ; D, lamprey, 

 diphycercal type ; £, Birkenia, hypocercal type, of which the details are unknown. 



different forms, both internal and external, and these are classified 

 into five main types. 



One would expect the earUest caudal fin to be symmetrical, 

 with the vertebral (or notochordal) axis running straight to its 

 tip, and with two equal expansions of membrane, one above and 

 one below, but such a tail is found neither in any early vertebrate 

 nor in any existing fish. It is, however, found in Branchiostoma, 

 in early fish embryos, and in the larvae of Amphibia. It is called 

 protocercal (Fig. 430). 



The oldest vertebrates had caudal fins which were strongly 

 asymmetrical. In the Placodermi, in the earliest elasmobranchs, 



