11 



in widtli jicconliii^" to llic food, tlie uiipcr sci-vice set witli iiiiinitc (Icnlicles 

 sharp-pointed in tlio insectivorous species, more or less ])lunl and conical 

 in those wiiich take a considerahle per centa^e of niollnscan food. The 

 immense develoijment of these structures in the slicepshead ( Aplodinotis) 

 as a crushinji' apparatus for luoUusca, is too well known to re(|uire descrip- 

 tion. In the Catostomatida' the number of teeth may vary from thirty 

 or less to two hundred (»r more, reduction in iurid)er yoi'iH' with increase 

 in size (especially in the lower part of the arch), botii bein^" related to an 

 increased importance of molluscan food. 



In the cyprinoids or minnow family this is i>ractically an inso<'t ivorous 

 a|)paratus, excejjt iti some of the species with very long intestine and llie 

 limophagous habit, where it seems useful chiefly as a means of grinding 

 up the mud ingested. 



In the piscivorous si)ecies and in those with highly developed gill-rakers, 

 the lower pharyngeals are commonly slight and insignificant, but in the 

 former group the upper pharyngeals may be preserved and enlarged as a 

 basis for the insertion of hooked teeth, to aid in the retention of their 

 struggling prey. 



Concerning the digestive structures, I will only remark that the fishes 

 with the longest intestine are mud-feeders, as a rule, and that in one of 

 them — the gizzard shad, a mud lover, ijar excellence — the pharyngeal jaws 

 (which in the mud-eating cyprinoids are evidently used to grind the food) 

 are functionally replaced by a bull)Ous, muscular stomach, the pharyngeals 

 themselves being reduced to thin and delicate plates, scarcely better than 

 rudiments. 



In this connection the adult size of the fisli ought always to be men- 

 tioned since this has, perhaps, at least as much to do witli the food as 

 any structural endowment, and frequently in fact has had a determining 

 inrtuence on the latter. Many fishes can enjoy the advantages of large 

 size only on condition that they acquire some new capacity of food pre- 

 hension adapting them to new food relations. Simple and symmetrical 

 growth of a small fish would render it incapable of straining out Ento- 

 mostraca without fitting it for the appropriation of any otlier food except, 

 perhaps, the larger Crustacea and some aquatic insects: and beyond this 

 insectivorous stage nothing is possible without new adaptations. 



COBRELATIONS OF ALIMENTARY OrGANS. 



Correlations of structure may be either mediate or immediate, in the lat- 

 ter case modification of one organ being directly dependent on modification 

 of another, and in the former both i)arties to the correlation l)eing mod- 

 ified 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 

 the incompatibility of canine teeth and heavy lower pharyngeals. The first 

 is an immediate adajjtive adjustment which a child might foresee, while 

 the others are to be understood only when the i)eculiarities of the food 

 are known to which both owe their charactei'. 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 apparatus of the shovel fish and canine teetii of the pike- 

 perch are examples of special adai)tation to particular kinds of food. 



Some mediate correlations are inverse, others coincident, the related 

 structures varying oppositely or in the same direction. An interesting in- 

 verse correlation is exhibited by the gill-rakers and the pharyngeals 

 in the suckers: as the former lengthen and multiplv. the latter become 

 weaker and l»ear smaller and more numerous teeth." The cause of this 

 correlation is seen in the food, the species with lieavv pharvngeals. few 

 and large pharyngeal teeth, and few and short gill-rakers being moUusk 

 7 F. C. 



