=Crustacean Parasites.=--The external parasites of fishes are of little injury. These are mainly lernæans and other crustaceans (fish-lice) in the sea, and in the rivers different species of leeches. These may suck the blood of the fish, or in the case of certain crustaceans which lie under the tongue, steal the food as it passes along, as is done by _Cymothoa prægustator_, the "bug" of the mouth of the menhaden (_Brevoortia tyrannus_). [Illustration: FIG. 226.--Australian Flying-fish, _Exonautes unicolor_ (Valenciennes). Specimen from Tasman Sea, having parasitic lernæan crustaceans, to which parasitic barnacles are attached. (After Kellogg.)] The relation of this crustacean to its host suggested to Latrobe, its discoverer, the relation of the "foretaster" in Roman times to the tyrant whom he served. A similar commensation exists in the mouth of a mullet (_Mugil hospes_) at Panama. The writer has received, through the courtesy of Mr. A. P. Lundin, a specimen of a flying-fish (_Exonautes unicolor_) taken off Sydney, Australia. To this are attached three large copepod crustaceans of the genus _Penella_, the largest over two inches long, and to the copepods in turn are attached a number of barnacles (_Conchoderma virgatum_) so joined to the copepods as to suggest strange flowers, like orchids, growing out of the fish. [Illustration: FIG. 227.--Black-nosed Dace, _Rhinichthys atronasus_ (Mitchill). East Coy Creek, W. N. Y. Showing black spots of parasitic organisms. (From life by Mary Jordan Edwards.)] =Myxosporidia, or Parasitic Protozoa.=--Internal parasites are very numerous and varied. Some of them are bacteria, giving rise to infectious diseases, especially in ponds and lakes. Others are myxosporidia, or parasitic protozoans, which form warty appendages, which burst, discharging the germs and leaving ulcers in their place. In the report of the U. S. Fish Commissioner for 1892, Dr. R. R. Gurley has brought together our knowledge of the protozoans of the subclass _Myxosporidia_, to which these epidemics are chiefly due. These creatures belong to the class of Sporozoa, and are regarded as animals, their nearest relatives being the parasitic _Gregarinida_, from which they differ in having the germinal portion of the spore consisting of a single protoplasmic mass instead of falciform protoplasmic rods as in the worm-like Gregarines. The _Myxosporidia_ are parasitic on fishes, both fresh-water and marine, especially beneath the epidermis of the gills and fins and in the gall-bladder and urinary bladder. In color these protozoa are always cream-white. In size and form they vary greatly. The cyst in which they lie is filled with creamy substance made up of spores and granule matter.