BULLETIN OF THE UNITED STATES FISH COMMISSION. 341 



by it« j;iv;it niiinber«, dcxvs serious hiirin, to free each fish from these 

 parasites, it is best to prevent their further increase by draining the 

 infected ponds, and to sell the fish before the gatherings in their skin 

 have become hirge, and to scatter a great (piantity of lime over the 

 bottom of the pond. 



The ArguluH foUaccus is round, pressed flat in the form of a shield, 4 to 

 (J millimeters [about \ inch] long, with along sucking-prong, two strong 

 sucking-disks with stems close to it, a pair of climbing legs with strong 

 claws, and four i)airs of webbed feet with stiff bristles. It has a dirty 

 greenish gray color, frequently swims about in the water in a lively 

 manner, and occasionally attacks young fish in such numbers as to 

 cover their entire surface, causing their death. The best means of de- 

 stroying them is to scatter lime in the ponds. 



Still greater than the number of crustaceans living on fish is the num- 

 ber of worms living on and in fish as parasites. But, as a general rule, 

 they do not do much harm, with the exception of the Piscicola geometra 

 L. This grows to the length of 2 or 3 centimeters [about 1 inch]. The 

 broad sucking-disks, located at the front and back part of the body, pro- 

 trude distincti}' from the thread-shaped body, which is only 1 to 2 milli- 

 meters [about j-g inch] broad, and of a greenish or yellowish-gray color 

 with darker bands. By means of these sucking-disks this worm moves 

 about in the manner of some caterpillars, while in the water it swims 

 about rapidly with a meandering motion. In ponds which contain 

 many of these worms the fish may be seen swimming about wildly, 

 covered with hundreds of these worms all over their bodies, principally, 

 however, round the fins, gills, eyes, and mouth. By rubbing against the 

 bottom and the banks, the fish try to rid themselves of these parasites 5 

 they grow lean, and frequently perish in large numbers. The best 

 means of destroying these worms is likewise to scatter lime in the 

 pond. By j)uttiug the fish for a sliort time in a solution of water and 

 1 per cent of common salt, they can be freed from these parasites and 

 again be placed in fresh water. 



Some plants likewise occasionally become hurtful in ponds. Reeds, 

 rushes, and other high aquatic plants should not be allowed to grow 

 too luxuriantly in the ponds. Sometimes green algse make their aj)- 

 pearance in such enormous quantity as seriously to impede the move- 

 ments of the young fish, by forming a net-work between other aquatic 

 plants. Sometimes also these algte cover a great portion of the surface 

 of the pond with a yellowish-green slimy mass. They can easily be 

 removed with a rake. 



Lower forms of algse, of the varieties Nostocliacece, Oscillarice, and 

 C/iroococcaceo?, occasionally produce by their astonishingly rapid growth 

 the so-called " water-bloom " ( Wasserhlilte)^ and transform the water 

 into a blue-green mass resembling oil. Sometimes this " water-bloom" 

 causes the death of all the fish in a pond; in other cases only certain 

 varieties die, and frequently the fish are not at all aftected by it, So 



