1058 FRESH-WATER BIOLOGY 



The dentition varies from none at. all to the crushing apparatus of 

 the white perch, the cutting incisors of some of the killifishes and 

 the rasplike patches of the teeth of the muscalonge. The mouth 

 varies in position, shape, and size according to the food, from the 

 ventrally-placed sucker mouth to the upward-pointing mouth of 

 Zygonectes; from the small mouth of the cisco to the capacious maw 

 of the muscalonge. The gill-rakers vary from none to the com- 

 plicated strainers of the spoonbill catfish. The alimentary canal 

 also varies with the food from the short canal of the flesh eaters 

 to the convoluted tube many times as long as the fish in the mud 

 eaters. That fishes are a very adaptable group is shown by the 

 fact that in South America a single family, the characins, have 

 the widest range of adaptation in the alimentary canal to different 

 food. Forbes has pointed out that the minnows of North America 

 are adjusted to a great variety of food. He distinguishes four 

 groups: (i) Intestine two to nine times as long as the fish, pharyn- 

 geal teeth not hooked, with grinding surface. (2) Intestine one to one 

 and two-thirds times as long as the fish, pharyngeal teeth hooked, 

 with grinding surface. (3) Intestine somewhat shorter than the fish, 

 teeth hooked, with grinding surface. (4) Intestine usually shorter 

 than the head and body, teeth hooked, without grinding surface. 

 Concerning the relation of these structures Forbes says: 

 "It is consequently from a comparison of the ratios of these 

 groups that we shall derive the most interesting facts relating to 

 the correspondence of food and structure. The most conspicuous 

 result is the great preponderance of mud in the intestines of the 

 fishes of the first group, characterized by an extraordinarily elon- 

 gate intestine, and by pharyngeal teeth destitute of hooks and pro- 

 vided with a broad grinding surface. Here, as already noted, 

 mud, sand, and gravel amount to about three-fourths of the mat- 

 ter ingested, while in the third and fourth groups only trivial and 

 accidental quantities occurred. In the second group, on the other 

 hand, with intestines intermediate in length, mud was still abundant, 

 but much less so than in the first; averaging less than half the 

 whole. If we exclude this indigestible matter, however, we shall 

 find the first group still further distinguished by the predominance 

 of vegetation as compared with animal matter, the latter being 



