330 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



sessed by nature. The Heterosomata can only be considered a 

 degenerate group. 



The double bony floor of the skull of the Distegous percomorph 

 fishes is a complication which places them at the summit of the 

 line of true fishes. At the summit of this division must be placed 

 the Pharyngognathi, which fill an imjoortant role in the economy 

 of the tropical seas, and the fresh waters of the Southern hemi- 

 sphere. By means of their powerful grinding pharyngeal ajipara- 

 tus they can reduce vegetable and animal food inaccessible to other 

 fishes. The result is seen in their multifarious species and innu- 

 merable individuals decked in gorgeous colors, and often reaching 

 considerable size. This is the royal order of fishes, and there is no 

 reason why they should not continue to increase in importance in 

 the present fauna. 



Very different is the line of the Plectognathi. The probable 

 ancestors of this division, the Epilasmia (Chaetodontidne, etc.), are 

 also abundant in the tropical seas, and are among the most brill- 

 iantly colored of fishes. One of their peculiarities is seen in a 

 shortening of the brain-case and prolongation of the jaws down- 

 ward and forward. The utility of this arrangement is probably to 

 enable them to procure their food from the holes and cavities of 

 the coral reefs among which they dwell. In some of the genera 

 the muzzle has become tubular (Chelmo), and is actually used as 

 a blow-gun by which insects are secured by shooting them with 

 drops of water. This shortening of tlie basicranial axis has pro- 

 duced a corresponding abbreviation of the hyoid apparatus. The 

 superior pharyngeal bones are so crowded as to have become a 

 series of vertical plates like the leaves of a book. These charac- 

 ters are further developed in the Plectognathi. The brain-case is 

 very small, the face is very elongate, and the mouth is much con- 

 tracted. The bones surrounding it in each jaw are co-ossified. 

 The axial elements (femora) of the posterior fins unite together, 

 become very elongate, and lose the natatory portion. In one group 

 (Orthagoriscidge) the jjosterior part of the vertebral column is lost 

 and the caudal fin is a nearly useless rudiment. In the Ostracion- 

 tidae (which may have had a different origin, as tlie pharyngeal 

 bones are not contracted) the natatory powers are much reduced, 

 and the body is inclosed in an osseous carapace so as to be capable 

 of very little movement. The entire order is deficient in osseous 

 tissue, the bones being thin and weak. It is a marked case of 

 degeneracy. 



