In many cases it is difficult to understand what is the advantage of the Hydroid to the animal that carries it, but in this last case Alcock[303] suggests that the _Stylactis_ assists in giving the fish a deceitful resemblance to the incrusted rocks of its environment, in order to allure, or at any rate not to scare, its prey. Whether this is the real explanation or not, the fact that in the Bay of Bengal and in the Laccadive and Malabar seas the fish is never found without this Hydroid, nor the Hydroid without this species of fish, suggests very strongly that there is a mutual advantage in the association. Cases of undoubted parasitism are very rare in this order. The remarkable form _Hydrichthys mirus_,[304] supposed to be a Gymnoblastic Hydroid, but of very uncertain position in the system, appears to be somewhat modified in its structure by its parasitic habits on the fish _Seriola zonata_. _Corydendrium {269}parasiticum_ is said to be a parasite living at the expense of _Eudendrium racemosum_. _Mnestra_ is a little Medusa which attaches itself by its manubrium to the Mollusc _Phyllirhoe_, and may possibly feed upon the skin or secretions of its host. Nearly all the species of the order are found in shallow sea water. _Stylactis vermicola_ and the "Challenger" specimen of _Monocaulus imperator_ occur at a depth of 2900 fathoms, and some species of the genera _Eudendrium_ and _Myriothela_ descend in some localities to a depth of a few hundred fathoms. _Cordylophora_ is the only genus known to occur in fresh water. From its habit of attaching itself to wooden piers and probably to the bottom of barges, and from its occurrence in navigable rivers and canals, it has been suggested that _Cordylophora_ is but a recent immigrant into our fresh-water system. It has been found in England in the Victoria docks of London, in the Norfolk Broads, and in the Bridgewater Canal. It has ascended the Seine in France, and may now be found in the ponds of the Jardin des Plantes at Paris. It also occurs in the Elbe and in some of the rivers of Denmark. The classification of the Gymnoblastea is not yet on a satisfactory basis. At present the hydrosome stage of some genera alone has been described, of others the free-swimming Medusa only is known. Until the full life-history of any one genus has been ascertained its position in the families mentioned below may be regarded as only provisional. The principal families are:—