42 A HISTORY OF FISHES 



case, during respiration the gullet is so tightly constricted 

 behind the last pair of pharyngeal clefts that little if any water 

 is able to find its way into the stomach. This closing of the 

 gullet is brought about by the action of special muscles en- 

 circUng the throat, which function in exactly the same way as 

 does a double string running round the neck of a bag. The pres- 

 sure of any food agairist the closed gullet, however, causes these 

 muscles to relax somewhat, and the solid nourishment is pressed 

 down into the stomach without the entrance of any water. 



In those fishes whose normal food consists of more or less 

 minute creatures swimming about in the water there is ob- 

 viously a danger of some of these escaping by way of the 

 pharyngeal openings and perhaps clogging or injuring in some 

 way the dehcate filaments. To lessen this danger special 

 structures known as gill-rakers have been evolved, taking the 

 form of a double row of stiff appendages on the inner margin 

 of each hoop-like gill-arch (Fig. i8). These, by projecting across 

 the pharyngeal openings, serve to strain the water which is to 

 bathe the gills, and to prevent any solid particles from passing 

 over with it. Generally the front row of rakers on each arch 

 interlocks with the hinder row on the adjoining arch, and the 

 two together form an eflTective sieve. In the Pike {Esox), 

 feeding almost entirely on other fishes, the gill-rakers are 

 represented merely by bony knobs, which may serve to block the 

 passage of larger food particles. In the fishes of the Herring 

 kind [Clupeidae), on the other hand, whose food consists of 

 minute shrimp-like creatures, the rakers are very numerous 

 and take the form of long, slender, and close-set bristles. In 

 some of the members of this tribe the filtering mechanism is 

 even more perfect, each primary gill-raker giving off secondary 

 and tertiary branches, the whole apparatus having the appear- 

 ance of the finest gauze. The form and number of the gill- 

 rakers may diflfer considerably even in two closely related 

 species, but this diflference may generally be correlated with a 

 difference in the normal diet. The two species of Shad (Alosa) 

 found in our own seas and rivers provide an excellent example 

 of this, the AUis Shad {A. alosa), having about eighty rakers on 

 the lower Umb of each arch in the adult fish, whereas the 

 Twaite Shad [A.jinta) has only thirty (Figs. i8a, b). The AUis 

 Shad feeds largely on small crustaceans, although it takes a 

 certain number of larval fishes. The Twaite Shad does not eat 

 the crustaceans to nearly, the same extent, but is much more 

 destructive to the young fry of other fishes. 



