4 



intermediate stages in others, and in still others is hardly discernible at 

 all. Moreover, the food choices of scarcely any fishes are so definite and 

 unchangeable as to be unmixed and identical under all conditions, in all 

 parts of their habitat, and at all times of the year. We shall find, indeed, 

 the same state of affairs when we come to deal with a habitat classifica- 

 tion; and if we look the whole field of ecology over we shall see it rather 

 characteristic of an ecological classification generally, especially on the 

 animal side. We can nevertheless profit greatly by such groupings of 

 our heterogeneous data as are still possible, if we admit the limitations 

 of the scheme and understand their significance. 



One of the most peculiar of our food habitat groups contains the 

 gizzard-shad and the stone-roller as its most notable representatives, to- 

 gether with a few minnows less strictly limited to it, all of them char- 

 acterized by unusually long, convoluted intestines, the gizzard-shad hav- 

 ing also the digestive surface still further increased by the development 

 of a very large number of finger-like caeca on its anterior section. These 

 fishes all discard intermediary agents, and help themselves to the raw ma- 

 terials of their food in the form of the mere mud and slime of the bot- 

 tom, which contains, of course, a considerable quantity of organic debris, 

 mostly of vegetable origin. They form the group of the mud-eaters. 

 (Figs. 2, 3, and 4.) 



The gizzard-shad, although but little eaten, is one of our most val- 

 uable fishes, since it is enormously abundant in our large waters ; both 

 rivers and lakes, competes with no other species for food, and is itself 

 the principal food of our game or predaceous fishes the most highly 

 valued products of our fisheries. It affords, also, a remarkable instance 

 of a transformation or development in the food habits and resources of 

 fishes, coincident with increase in size. From the time this fish hatches 

 from the egg until it comes to an inch or so in length it is as slender as a 

 minnow, with the alimentary canal a simple straight tube. Still more 

 remarkable, although the mouth of the adult is perfectly toothless, the 

 young have, at this stage, a row of conical, pointed teeth upon the upper 

 jaw. (Fig. 5.) Teeth would evidently be useless to it in sucking up 

 mud or straining out plankton from the water; but to the larva if such 

 it may be called they must be very useful, for instead of being a mud- 

 eater the fish is predaceous in this stage, its prey being the minute animals 

 of the plankton, especially the Entomostraca, which it pursues and cap- 

 tures one by one as a pike might capture minnows. With its growth and 

 transformation it changes its habits slowly, its food becoming more mixed 



