xx BRITISH FRESHWATER FISHES 



and in the Eel they are quite small. When the gill- 

 covers are lifted up the four gill-arches are seen, 

 separated by clefts which lead into the pharynx, and 

 each bearing two rows of red gill-filaments ; the inner 

 edges of the gill -arches are usually provided with 

 gill-rakers, stiff appendages which vary in length and 

 number according to the nature of the usual food, 

 their object being to strain the water which has 

 access to the gills and to keep the food from passing 

 through with it. Thus in the Pike the gill-rakers are 

 mere knobs, but in the Allis Shad they are close-set 

 bristles, very long, slender, and numerous. The gill- 

 arches are supported by a special branchial skeleton, 

 and their gill -bearing portions are divided into upper 

 and lower limbs which form an acute backwardly 

 directed angle at their junction, and behind the lower 

 limbs of the last arch and parallel with them lie the 

 lower pharyngeals, a pair of dentigerous bones which 

 are all that remains of a former branchial arch ; these 

 usually bite against the toothed upper pharyngeals, 

 or upper elements of the preceding arches, but in the 

 Carp family they work against a pad on the base of 

 the skull, and in this group the form, number, and 

 arrangement of the pharyngeal teeth are of great 

 systematic importance. 



The scales covering the body are usually arranged 

 in more or less regular, parallel, oblique, and longi- 

 tudinal series ; as a rule they are imbricated, so that 

 only the posterior part of each is exposed. In most 

 species they are cycloid, i.e. smooth and with the 

 edges entire, but in the Percoids they are ctenoid, i.e. 

 the edges are ciliated, as in the Bass, and in addition 

 the surface may be roughened with little spines, as in 



