TO AQUATIC LIFE. 59 



the embryos of higher vertebrates ; and fishes have been 

 called, in the language of the transmutation-of-species 

 hypothesis, "arrested gigantic tadpoles." It will be 

 found, however, that so far from there having been any 

 stoppage of development, the branchial arches have been 

 adapted to the exigencies of the fish by advancing to a 

 grade of structure which they never reach in the frog. 

 This is shown by their firm ossification, and their numer- 

 ous elastic joints ; the sieve-like valves developed from 

 the side next the mouth have been pre-arranged, with the 

 utmost complexity and nicety of adjustment, to prevent 

 the entry of any particles of food, or other irritating mat- 

 ters, into the interspaces of the tender, vascular, and 

 sensitive gills. It is interesting, also, further to observe, 

 that the last pair of these arches, which, when the em- 

 bryo-fish is as yet edentulous, usually support gills, are 

 reduced, when the supply of yolk-food is exhausted, and 

 the jaws get their prehensile organs, to the capacity of 

 the gullet, become thickened, in order to support teeth 

 for tearing in pieces, mincing, or crushing the food, and 

 are converted into an accessory pair of jaws, and this 

 pair the most important of the two, as it would seem ; for 

 the carp-tribe — e. ^., tench, barbel, roach — which have no 

 teeth on their proper jaws, have teeth on the pharyngeal 

 jaws. In no other vertebrate animals, save the osseous 

 fishes, is the mouth provided with maxillary instruments 

 at both the fore and hind apertures ; and in no other part 

 of the piscine structure is the direct divergence from any 

 conceivable progressive scale of ascending organisms, 

 culminating in man, so plainly marked as in this. 



The general form of the fish is admirably adapted to 

 the element in which it lives and moves. The viscera 

 are packed in a moderate compass, in a cavity brought 



