502 FISHES. 



zones. They frequent brackish waters, in which they find an 

 abundance of food which consists chiefly of the organic sub- 

 stances mixed with mud or sand ; in order to prevent larger 

 bodies from passing into the stomach, or substances from 

 passing through the gill-openings, these fishes have the organs 

 of the pharynx modified into a filtering apparatus. They 

 take in a quantity of sand or mud, and, after having worked 

 it for some time between the pharyngeal bones, they eject the 

 roughest and indigestible portion of it. The upper pharyn- 

 geals have a rather irregular form ; they are slightly arched, 

 the convexity being directed towards the pharyngeal cavity, 

 tapering anteriorly and broad posteriorly. They are coated 

 with a thick soft membrane, which reaches far beyond the 

 margin of the bone, at least on its interior posterior portion ; 

 this membrane is studded aU over with minute horny cilia. 

 The pharyngeal bone rests upon a large fatty mass, giving 

 it a considerable degree of elasticity. There is a very large 

 venous sinus between the anterior portion of the pharyngeal 

 and the basal portion of the branchial arches. Another mass 

 of fat, of elliptical form, occupies the middle of the roof of the 

 pharynx, between the two pharyngeal bones. Each branchial 

 arch is provided on each side, in its whole length, with a 

 series of closely-set gill-rakers, which are laterally bent 

 downwards, each series closely fitting into the series of the 

 adjoining arch ; they constitute together a sieve, admirably 

 adapted to permit a transit for the water, retaining at 

 the same time every other substance in the cavity of the 

 pharynx. 



The lower pharyngeal bones are elongate, crescent-shaped, 

 and broader posteriorly than anteriorly. Their inner surface 

 is concave, corresponding to the convexity of the upper 

 pharyngeals, and provided with a single series of lamella, 

 similar to those of the branchial arches, but reaching across 

 the bone from one margin to the other. 



