286 PRINCIPLES OF ANIMAL BIOLOGY 



of Faston's aoeount of Lerncpopodn edwardsii, a copepod parasitic on th 

 gills of the brook trout, Salvelinusfo7itinalis, in Wisconsin and elsowh(^r( 

 This parasiti(! copepod hatches from the egg as a free-swi mining larvj 

 form which immediately by swimming with a darting spiral motion begir 

 a search (?) for a proper host. Its body has a high specific gravity 

 hence it tends to settle to the bottom when the animal is not activel 

 swimming. In strong light, sunlight or artificial, the copepod swirr 

 near the surface of the water or toward the light, because it is positivel 

 heliotropic. To weak light its reaction is less marked, and if the ligl 

 is very weak, as at night, the copepod gives no reaction at all and becaus 

 of its high specific gravity it sinks to the bottom. Its behavior to ligl 

 is not interfered wuth by most of the chemical substances used in exper 

 ments; but if into a dish containing larval copepods, the freshly excise 

 gills of the brook trout are placed, the copepods dart about as if the 

 sensed something in the water. If they come in contact with the gil 

 they attach themselves at once. If in place of the gills of brook trou 

 gills of rainbow trout are substituted the copepods do not attach. Th 

 latter effect is also obtained if gills of German-brown trout are usee 

 Evidently something diffuses from the gills of the brook trout which doc 

 not come from the other species of trout mentioned. This must be 

 chemical substance. Observations on fish from the ponds show tha 

 only brook trout are infested by this parasite while the two other specie 

 of trout mentioned, as well as suckers and perch, are immune. Th 

 copepod gives a positive chemotactic response to the gills of the broo 

 trout and not to the other fish. 



Now, in the ponds during the day time the parasitic copepod swin 

 near the surface in response to light; so also do the young trout, probabl 

 in response to light stimulus but also because of the presence of foo 

 organisms which likewise are positively heliotropic. At night the troi 

 seek the bottom; and the parasite, because it does not I'ospond to wea 

 light and because of its specific gravity, likewise goes to the botton 

 Again parasite and host are brought into close quarters. While th 

 proximity of parasite and host is advantageous to the parasite, it is nc 

 deliberately sought. It is the animals' responses to certain stimu 

 which bring this parasite into relation with its animal environment. 



Numerous other examples in support of the general conclusion th? 

 responses to stimuli determine the habitat or immediate environment ( 

 animals might readily be found. Sometimes these responses lead th 

 animal to an advantageous situation, and may then be described £ 

 adaptive. Often, on the contrary, the reaction appears to be of n 

 benefit whatever. The response cannot, however, lead to very harmfi 

 conditions, for the animals would then perish. Probably, in the evoh 

 tion of animals, such ill-designed reactions have repeatedly originatec 

 but if they did, their possessors perished, and we have left today onl 



