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. 
Ledermuller 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 wishing again 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 immediately out- 
side ; and Dana and Herrick inform us that their Argulus 
Catostomi was always 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 
