FINS 



in exactly the same manner, but, whilst the upper lobe has 

 been reduced to a mere rudiment, the lower lobe is enor- 

 mously developed and forms the greater part of the fin. The 

 upturned part of the backbone is generally converted into a 

 single bone, known as the urostyle, 

 and the rays of the caudal fin are 

 attached to the lower spines of the 

 hinder vertebrae, which are greatly 

 enlarged and at the same time in- 

 clined backwards so as to be more 

 or less parallel to the axis of the 

 body (Fig. 26b). 



In some of the more specialised 

 forms certain modifications of the 

 homocercal tail occur. The term 

 "leptocercal" (leaf tail) is appHed 

 to those tails of an attenuate or whip- 

 like form, in which the vertebral 

 column tapers to a fine point, as in 

 the Grenadiers or Rat-tails {Macruri- 

 dae) (Fig. 62a), and in some of the 

 Blennies [Blenniidae) . In other fishes 

 the upturned part of the backbone 

 is scarcely recognisable, and the fin- 

 rays seem to be equally derived from 

 the upper and lower lobe, resulting 

 in a superficially diphycercal tail. In 

 others, again, an apparent diphycercy 

 is produced by the atrophy of the 

 hinder part of the vertebral column 

 in the adult, the upper and lower 

 lobes of the caudal fin coalescing 

 round the end of the abbreviated 

 tail (Fig. 26d). In the Cods and 

 their allies [Gadidae) occurs yet 

 another modification, known as the 

 isocercal (equal tail) type. This is 

 actually, and not only apparently 

 symmetrical, consisting of equal numbers of fin-rays separated 

 by the axis of the vertebral column, \vhich is continued straight 

 backwards, the separate vertebrae growing progressively smaller 

 behind (Fig. 26c) . This symmetry has clearly been secondarily ac- 

 quired, and has resulted from the loss of the original hetero- 



Fig. 27. DEVELOPMENT OF 



CAUDAL FIX. 



Six Stages in the develop- 

 ment of the tail region in 

 the American Flounder 

 {Pseudoplciironectes avierican- 

 us). (After Agassiz.) 



