THE INFANCY OF FISHES 229 



while the tail fin shortens. Further, both in coloration 

 and in the shape of the breast fins the larva differs con- 

 spicuously from the adult, in the young stage these fins 

 are used after the fashion of fore-legs, to support the 

 body. Finally, attention must be drawn to the external 

 gills, which recall those of amphibia. These are, as we 

 have already seen, confined to larval life, carrying on 

 the task of breathing till the adult gills are sufficiently 

 advanced to take up their work : though in many cases 

 they also serve to absorb nourishment from the yolk, 

 and thus combine the duties of feeding and digestion ! 



While some of the most striking of the peculiarities of 

 young fishes are those which belong solely to larval life, 

 and have been developed to meet its needs, others, un- 

 questionably, answer to phases of earlier, ancestral, adult 

 phases of growth. To enlarge upon this aspect of young 

 fishes would involve us in technicalities, but the nature 

 of the evidence can readily be grasped from an examination 

 of the tails of one or two of the larvae which we have 

 already surveyed. 



Take the young Angler, for example. In the earliest 

 stage it will be noticed in the figure that the tail forms a 

 continuous line with the rest of the body, and has no fin ; 

 in the second stage rudiments- of the fin have appeared ; 

 while in the third stage, it will be noted, the end of the 

 tail turns suddenly upwards, bearing a narrow marginal 

 fin along its upper border and a much larger fin on its 

 hinder border. Here we have what is known as a 

 diphycercal tail, characteristic of adult sharks. Now in 

 all the more specialised or so-called " bony " fishes, 

 such as the angler, and the herring, salmon, cod, and so 

 on, the tail fin, as any one can see for himself who 

 will take the trouble to look, is fan-shaped, the bony 



