128 THE STORY OF FISH LIFE. 



pendant as in the shark, but the same curious 

 history with regard to the closing of the gullet is 

 repeated here. 



This matter of the relation between the larva 

 and the egg is somewhat of a digression, albeit 

 necessary. Besides the circular suctorial mouth 

 disc — which, however, as we have seen, may be 

 represented only in part, either by that portion 

 in front of, or behind the mouth, or more primi- 

 tively still, by a suctorial mouth — and the 

 external gills, our larval form is conspicuous 

 for the absence of limbs, and the presence of a 

 long tail fringed by a delicate membrane, the 

 tail-fin. This tail is the only organ of propul- 

 sion. Such a form is one of peculiar interest, and 

 of first-rate importance, since it is characteristic 

 of many very different, and only distantly related, 

 animals. Occurring as a phase in the life history 

 of the tunicates or sea-squirts, fishes and amphibia. 

 This fact is regarded by scientific experts as a 

 reasonable proof that these early stages, common 

 to such different forms, represent the primitive 

 vertebrate model out of which all the vertebrata 

 have grown by gradual modifications, and 

 transformations. The nature of these trans- 

 formations we have already hinted at. Thus, 

 to take the breathing-organs. These are first 

 represented by external gills, which are gradually 

 rei)laced by internal gills, whose duties are in 

 great part transferred, in some fishes, to still 

 more internally removed respiratory organs, 

 which we call lungs. In the frog tadpole ex- 

 ternal and internal gills each in turn pass away, 

 and are completely and slowly supplanted by 



