252 BRITISH ENTOMOSTRACA. 



enters, and renders, consequently, brackish, so that it 

 would appear this little animal, though in reality a fresh- 

 water species, is not injured by an admixture of salt. 



The Argulus foliaceus is an exceedingly pretty and 

 graceful little animal ; and as it can leave the fish upon 

 which it feeds, and swim freely in the water, there are 

 many opportunities for watching its gambols through its 

 native element. It generally swims in a straight line, but 

 it frequently suddenly changes its direction, and often 

 turns over and over several times in succession. 



Ledermiiller says he has seen an individual turn over 

 thus a hundred times in a minute, and that it swam after- 

 wards with such velocity, sometimes skimming the sur- 

 face, at others plunging deeper in the water, that he could 

 scarce follow its motions with his eye. Sometimes it will 

 attach itself to the side of the vessel in which it is kept; 

 and we may then observe that though, as far as progres- 

 sive motion is concerned, the little creature is at rest, the 

 natatory feet are in constant motion. Like those of the 

 Branchiopoda, they serve the animal not only for swimming 

 with, but also for respiration. Their structure is pre- 

 cisely similar, and their use the same. When swimming 

 free in the water, and wishino- ao-ain to fasten itself to its 

 prey, the little animal approaches a fish, quietly allows 

 itself to be hurried along in the current caused by its 

 motion through the water, till it touches it, when it im- 

 mediately fastens itself to it, choosing, according to 

 Jurine, as its place of residence the under part of the pec- 

 toral fins. 



Loefling states that the part where the Argulus foliaceus 

 is chiefly found is within the gills, or innnediately out- 

 side ; and Dana and Herrick inform us that their Argulus 

 Catostomi was alwags found within the branchial cavities ; 

 but when the fish itself was immersed in fresh water the 

 parasite forsook the gills, and, after swimming about for 

 some time, would often attach itself to the anterior part 

 of the body. 



Jurine observed that the fishes seemed to be afraid of 



