TEETH AND FOOD 135 



(Cichlidae) of the Great Lakes of Africa all kinds of dentitions 

 have been evolved in course of time: the vegetarians have bands 

 of small, notched teeth in the jaws, sometimes with an outer 

 series of chisel-like incisors for cutting weeds, the fish-eaters 

 have the large mouth armed with strong, pointed teeth, and 

 those which live largely on molluscs have strong, blunt, 

 pharyngeal teeth. , . ,. i u 



Many fishes habitually include in their diet large numbers 

 of larvae of aquatic insects, and flies, gnats, and the like, 

 flying near the surface of the water are also seized and devoured, 

 some fishes displaying great agility in leaping out of the water 

 and securing the prey at a single snap. The Archer-fishes 

 (Toxotes), found on the coasts and in the rivers from India to 

 the Pacific, derive their name from the curious manner in which 

 they obtain the insects on which they feed (Fig. 57). Observing 

 a fly hovering near the surface or settled on weeds or grass, 

 the Archer carefully approaches, and, taking careful aim, 

 squirts a drop or two of water from its mouth at the victim, 

 which falls in the water and is soon secured. Their aim is very 

 accurate even at a distance of three feet. 



Many fish, of which the famiUar Herring {Clupea) is an 

 example, are plankton feeders; that is to say, they live exclusively 

 on the swarms of microscopic organisms, both animal and 

 vegetable, swimming at or near the surface of the sea and 

 constituting what is known as the plankton [cf, p. 407). Like 

 the land vertebrates, all fishes are ultimately dependent on the 

 material and energy suppHed by plants for their existence and 

 these plants in their turn are dependent on the rays ot the 

 sun to turn non-living materials into living substances, ^or 

 example, the little crustaceans known as Copepods feed on the 

 microscopic plants called Diatoms, the plankton-feeding hsh 

 devour the Copepods, and are themselves eaten by more 

 powerful fishes. That the plankton provides a sufficiency ot 

 nourishment is proved by the fact that the huge Basking Shark 

 iCetorhinus) is able to subsist exclusively on such a diet ((/.p. 43) • 

 Diatoms hkewise form an important part of the food ot bottom- 

 living animals like the molluscs, echinoderms, and worms so 

 that fishes feeding on these invertebrates are again dependent 

 in the long run upon the vegetable kingdom. As might be 

 expected from the nature of the food, the teeth of the Herring 

 (Clupea) are small and feeble, and the food is strained from the 

 water by the filtering mechanism provided by the slender 

 gill-rakers [cf. p. 42). The Hickory Shad [Dorosoma] of 



