508 Illinois State Laboratory of Natural History. 



ally replaced by a bulbous, muscular stomach, the pharyngeals 

 themselves being reduced to thin and delicate plates, scarcely 

 better than rudiments. 



In this connection the adult size of the fish ought always 

 to be mentioned, since this has, perhaps, at least as much to do 

 with the food as any structural endowment, and frequently, in 

 fact, has had a determining influence on the latter. Many 

 fishes can enjoy the advantages of large size only on condition 

 that they acquire some new capacity of food prehension, adapt- 

 ing them to new food relations. Simple and symmetrical 

 growth of a small fish would render it incapable of straining 

 out Entomostraca without fitting it for the appropriation of 

 any other food, except, perhaps, the larger Crustacea and some 

 aquatic insects; and beyond this insectivorous stage nothing is 

 possible without new adaptations. 



CORRELATIONS OF ALIMENTARY ORGANS. 



Correlations of structure may be either mediate or imme- 

 diate, in the latter case modification of one organ being directly 

 dependent on modification of another, and in the former both 

 parties to the correlation being modified by a common 

 cause. The immediate class of correlations are rela- 

 tively few and simple in the alimentary structures of fishes, 

 while several of the mediate class are less obvious and more 

 suggestive. That a fish with canine teeth has a strong jaw is 

 a less interesting fact than the weakness of the jaw in one with 

 long and numerous gill-rakers, or the incompatibility of canine 

 teeth and heavy lower pharyngeals. The first is an immediate 

 adaptive adjustment which a child might foresee, while the 

 others are to be understood only when the peculiarities of the 

 food are known to which both owe their character. The weak jaw 

 of the shovel fish and the slight lower pharyngeals of the pike- 

 perch illustrate the law of disuse (especially when we take into 

 account the teeth of the young in the former and the large 

 pharyngeals of the common perch), and the branchial appar- 

 atus of the shovel fish and the canine teeth of the pike-perch are 

 examples of special ada|)tation to particular kinds of food. 



