62 A HISTORY OF FISHES 



cereal tail and the meeting of the dorsal and anal fins round 

 the hinder end of the fish. In other words, the tail-fin of the 

 Cod is not a true caudal fin at all, being composed in the main 

 of dorsal and anal rays. 



A study of the embryonic development of the tail provides 

 another interesting example of that recapitulation which has 

 been already mentioned {cf. p. 55). In the tail of a larval 

 American Flounder (Pseudopleuronectes) , for example, in the 

 very young fish this is without fin-rays and is truly diphycercal 

 (Fig. 27 1). The hinder end of the vertebral column soon 

 commences to turn upwards, numerous hair-like rods make 

 their appearance along the lower edge of this axis, and the tail 

 becomes definitely heterocercal (II, III). Later, the terminal 

 portion becomes more bent, the upper lobe degenerates, and 

 the lower lobe increases enormously in size, is directed back- 

 wards, and finally forms a superficially symmetrical tail (IV, 

 V, VI). Thus, during the lifetime of a single individual the 

 tail is transformed from a perfectly symmetrical organ through 

 a distinctly asymmetrical stage into a superficially symmetrical 

 one, all three types being represented in the order in which 

 they were originally evolved. A study of the evolutionary 

 history as revealed by the record of the rocks provides a similar 

 story. The earliest known forms had a diphycercal tail, but 

 very soon the heterocercal type appeared, with a very slight 

 upward bend of the backbone and a small dorsal and large 

 ventral lobe. Such a type of tail is found in all the ancient 

 Selachians and Sturgeon-like fishes, and persists to-day in their 

 living descendants. The homocercal tail, however, also made 

 its appearance at an early stage, and is found in a fish {Dapedius) 

 known to have flourished during the epoch known as the Lower 

 Lias. Other curious forms of the Triassic and Liassic periods 

 actually had two tails at one time. Of these, Diplurus and 

 Undina (Fig. 129) are of special interest, exhibiting the true 

 tail very much reduced in size and appearing as a tiny ap- 

 pendage bearing a minute caudal fin. In front of this is another 

 fin, in all probability the functional tail-fin, made up entirely 

 of rays derived from the hinder parts of the dorsal and anal. 

 If these enlarged dorsal and anal rays increased still further in 

 size and eventually superseded the true caudal fin the secon- 

 darily symmetrical tail of the Cods would come into being. 



Having outlined the main facts concerning the origin and 

 structure of the fins, it will be convenient to examine some of 

 the more interesting modifications which these organs have 



