THE PHARYNGEAL TEETH OF FISHES 169 



at least 210 long, thin, horny gill-rakers ; these stand on a 

 length of two and five-sixteenths of an inch, so average just 

 over ninety-one to the inch ; the longest one is one-half longer 

 than the gill-lamina helow it ; the gill-rakers are smooth, deli- 

 •^■ate, easily broken off. They are equally thickly set along the 

 ijpibranchial, which in this fish is a longer bone than usual, its 

 length being about two-thirds that of the cerato-hypobranchial. 

 The gill-rakers spring from the top surfaces of the branchial 

 arches, and there is only one row of them. The gill-rakers of 

 the other arches are of the same kind, but not so long as those on 

 the first arch ; they diminish in length as they grow on the 

 arches nearer the centre of the mouth. The top edge of the 

 first arch is at a higher level than that of the second, and so on, 

 so that the cavity of the mouth resembles a deep basket. The 

 gill-rakers of one arch fit up against the side of the next outward 

 arch, and the whole apparatus forms a perfect filter. The fifth 

 branchial arch, the pharyngeal bones, are devoid of teeth ; and 

 instead of lying flat at the bottom of the mouth, they are 

 inclined to one another at an angle upwards ; the object of this 

 will be seen presently. From the surface of the first branchial 

 arch to the floor of the mouth is a depth of about three-fourths 

 of an inch. The epibranchials and the connected tissues 

 form a mass, giving the general idea of the plug of a wooden 

 lemon-squeezer ; in this mass, on the middle line in front, 

 is a deep cleft ; this fits on the upstanding angle of the 

 pharyngeal bones ; the action is to squeeze the water in the 

 buccal cavity to the right and left, forcing the water through the 

 gill-rakers, leaving the particles of food, which are minute, in 

 the gullet. To aid in this action there is a pronounced flange 

 under the parasphenoid bone that reinforces the squeezing mass. 

 Engraulis encrasicholus, the Anchovy, has thirty thin, horny 

 gill-rakers on the cerato-hypobranchial of the first arch, with 

 twenty on the epibranchial. 



Gymnotid^, 



Sternopygus macrurus, one of the Gymnotidce from British 

 Guiana, has five tubercle gill-rakers on the first cerato-hypo- 

 branchial arch, and one developed with two rudimentary on the 

 first epibranchial. There are gill-rakers on the inside of the 



