LARVAL FISHES : THEIR METAMORPHOSES. 125 



itself to a rock by this disc, and remains fixed 

 whilst it undergoes the very extraordinary and 

 remarkable changes which ultimately end in its 

 transformation into the shapeless mass from 

 which the adult form takes its name. The 

 curious tactile organs or barbels, described else- 

 where, are probably structures arising from the 

 modification of this disc. The larval sturgeon 

 shows how this has came about. 



We may turn now from the mouth to a con- 

 sideration of the gills of the larval fish, since in 

 these we have again characters which are shared 

 in common with their allied but less humble 

 relatives, the amphibia. In the young shark, 

 and, to a certain extent, the sturgeon, and in the 

 young bony-pike (Lepidosteous) mud-fish (Pro- 

 topterus), and " bichir " (Polypterus) fig. 3, p. 26, 

 the breathing organs or gills, like those of the 

 larval frog or newt, take the form of more or 

 less branched or feather-like organs, the branches 

 springing from a common shaft, or of delicate 

 filaments projecting from the gill-slits. 



It is to be noted, however, that it is the so- 

 called " ganoid-fish " and lung-fish larva which 

 most nearly resembles the amphibian : the deli- 

 cate filamentary gills of the shark, it is believed, 

 probably represent secondary and not primary 

 structures. These gills are in all cases but tem- 

 porary outgrowths, being replaced in the fishes 

 either by internal gills which have already been 

 described (p. 20) or by gills and lungs, e.g. : 

 lung-fishes, or by lungs only, e.g. : amphibia. 

 These permanent breathing organs, it appears, 

 require time for growth, hence the temporary 



