126 THE STORY OF FISH LIFE. 



Structures which gradually become absorbed in 

 proportion as the permanent structures assume 

 their duties. In the case of some of the 

 amphibia, e.g. : the axolotl of Central America, the 

 larval condition — and hence the external gills 

 also — is rarely exchanged for the adult form, all 

 the functions of life including the reproduction 

 of the species being fulfiUed by the larva. The 

 young of the higher (Teleostean) fishes never 

 produce external gills. 



Besides the suctorial mouth disc and the ex- 

 ternal gills, we have yet another larval character, 

 one which not only carries us back to, but 

 actually connects the larva with, the egg itself. 

 The larval condition, as we have already re- 

 marked, is consequent upon the fact that the egg 

 from which the young is produced contains only 

 a limited amount of food material insufficient to 

 enable the growing embryo or unhatched fish, to 

 complete its growth into a fully formed fish. For 

 this reason, as soon as the process of hatching 

 has become an accomplished fact, certain tempo- 

 rary structures have to be developed in order that 

 the processes of further development may be 

 completed. The nature of some of the more 

 important of these temporary structures we have 

 just discussed. Some of them undergo further 

 transformation and development into adult struc- 

 tures, and some are purely larval organs and are 

 put away with other childish things, if we may 

 be permitted the metaphor, when the adult stage 

 is reached. The newly-hatched young of the 

 shark tribe will best bring home the nature of 

 the relation between the larva and the egg. 



