Parasites are organisms that live upon other organisms, whether plants or animals. Those that merely live in company with others, and do not obtain their food from the organism with which they dwell, are known as commensals. Parasitism is very widely spread in the animal kingdom, and it has representatives in most groups, especially of the Invertebrata. Nearly all animals are subject to their attacks. Among the Protozoa, numerous Infusoria, such as Trichomanes and most of the Gregarines (q.v.), which live in the lungs of frogs and the digestive tube of worms, are parasitic. There are few representatives among the Ccelenterata or Echinoderms, hut the worms include many of the most destructive. Thus all the Entozoa are parasitic, including the tapeworms, liver-flukes, and the tropical parasites Bilharzia and Filaria, which give rise to hematuria. Among other classes of worms, parasites occur in the Nematoda, such as those which cause grouse disease (Strongylus peryracilis) and the "gapes" of poultry (Sclerostoma synyamus); also in the Cha-Btopoda, where the family Myxostomidce burrow in the stems of crinoids, and the genus Oligognathus lives in a Gephyrean. But few cases occur among the Mollusca, and the principal one is Stylifer, which is parasitic on starfish. The Arthropods supply many and interesting cases; thus in the Barnacles the males are parasitic on the females. There are many parasitic insects, such as the too well-known lice, fleas and bugs. Among the Arachnida the chief examples are the Ticks and Mites, the Pentastomida, which live in the nasal cavities of dogs, etc. Among the Crustacea there were two orders once recognised which included only parasites, these were the Ichthyophthiraand Rhizocephala; but the orders are now abandoned, as it is known that the former included parasitic Copepods, and the latter parasitic Cirripedia. The history of these two orders illustrates the degeneration that always ensues when animals take to this mode of life. The only case of parasitism in Vertebrates is in the case of two genera of fishes forming the family Myxinoidei; the better known of these is Myxine, the Hagfish.