SILURID^E THE CATFISHES 175 



mere natural waste, and, in so far as they are themselves eaten by 

 other fishes, they thus increase the general supply of fish food in the 

 waters they enter and inhabit. By their -services as scavengers, 

 they help to protect more sensitive fishes from the effects of the pol- 

 lution of the water through a decomposition of objects which they 

 are themselves very willing to devour, and in this way also they may 

 convert into a form acceptable to other fishes food substances other- 

 wise useless. As we have found them to be eaten more or less by 

 both our species of black bass, by the sand-pike (Stizostedion cana- 

 dense), and by the yellow bullhead and the mud-cat, their utility in 

 this sense seems appreciable. 



On the other hand, it must be noticed that they have appeared 

 yery rarely in the food of fishes, in comparison with their numbers 

 and general distribution. Only nine fishes out of more than 1,200 

 examined had eaten them, while 45 of these, same fishes had eaten 

 more or less freely of a single species of another family — the 

 gizzard-shad. Reviewing the food of the eatfishes themselves, it 

 seems to us clear, from our present data, that they devour other 

 fishes much more generally than others devour them — that whatever 

 tends to their multiplication and continuance tends rather to dimin- 

 ish the number of other species in our waters than to increase them. 

 Their partial immunity is doubtless due in considerable measure to 

 their remarkable defensive apparatus of stiff, acute, projecting, 

 poisoned spines in the pectoral and dorsal fins, weapons capable of 

 inflicting really painful punctures in animals" as large as man. These 

 fin-spines are evidently an advantageous substitute for the defensive 

 armor of scales which our eatfishes have lost, in the course of their 

 evolution. 



The nocturnal habits of eatfishes must also contribute to their 

 protection from predaceous enemies, and the wide range of their 

 dietary enables them to exist in much larger numbers than would be 

 possible if their choice of food were more restricted. Where one 

 kind fails them for a time they may find an abundance of another. 

 Their power to crush the shells of many mollusks and to reject the 

 fragments gives them access to a means of subsistence very abun- 

 dant in many of the waters which they inhabit, and available to but 

 few other fishes, and their habit of leading and guarding their young 

 of course greatly increases their chances of survival. 



Our eatfishes are not by any means all of equal habit, or of similar 

 distribution and ecological relationship. The stonecats remain the 

 size of minnows and the channel-cats are among the heaviest of 

 .the fishes of our great rivers. The former lurk, like darters, under 



