78 NATURE STUDIES. 



so to speak who " dine out/' but who repose within 

 the anatomical establishment of a " host." This is 

 the case with certain little fishes, which choose the 

 very " jaws of the lion" as a dwelling-place, since 

 they appear to live in the interior of certain big 

 tropical sea-anemones. These fishes may be seen to 

 swim in and out of the anemone's mouth, and they 

 may be enclosed within the anemone's body when 

 that animal contracts itself, and yet swim free and 

 unharmed out of the mouth when these flower-like 

 animals once more resume their normal and expanded 

 state. Here, then, there is mere " association;" but 

 it is in some such association that the beginnings of 

 pure parasitism have originated. Suppose the case of 

 an animal which, at first merely " lodger," took to 

 feeding upon the tit-bits secured by its host for home 

 consumption. The " lodger," in such a case, would 

 practically become a " boarder" as well. But nature 

 has a law as fixed as the edicts of the Medes and 

 Persians, called the " law of disuse." This law enacts 

 that whatever structures or organs of living beings 

 are not normally used, will waste and tend to disappear. 

 It is the operation of this law which has caused the 

 two outer toes of our horse to grow " small by degrees 

 and beautifully less," until they now appear as the 

 " splint bones " on each side of the single toe upon 

 which the horse walks. And applying this law to the 

 case of the animal lodger, we see how an animal which 

 does not require to move about when resident within 

 another animal will lose its organs of motion. If it 



