The order Isopoda includes a very large number of parasitic species. The extensive family Cymothoidæ presents a whole series of gradations in habits and structure between actively swimming predatory species and others which in the adult state are permanently fixed to their host, usually a fish, and are incapable of movement. At one end of the series are the species of _Cirolana_, which have powerful biting jaws. Of _C. borealis_ (Fig. 70), Mr. Stebbing remarks that "it is a good swimmer, tenacious of life, a savage devourer of fish, and not to be held in the human hand with impunity." The species is not uncommon in British seas, and numerous individuals will sometimes attack a Cod or other large fish, perhaps after it has been caught on a hook, and gnaw their way into its body, so that when brought to the surface the fish consists of little more than skin and bone. [Illustration: FIG. 70--_Cirolana borealis._ ABOUT TWICE NATURAL SIZE. (After Sars.)] The little _Eurydice achatus_, belonging to the same subfamily, Cirolaninæ, is commonly taken in the tow-net in sandy bays on our own coasts. It is said sometimes to attack bathers, and to "nip most unpleasantly." More definitely parasitic are the species of _Æga_ and allied genera, which have piercing and suctorial mouth parts, and suck the blood of fish. They are usually found adhering closely to the skin of their victim by means of the strong hooked claws of the anterior pairs of legs; but they have not lost the power of locomotion, and, as females bearing eggs are never taken on fish, it would appear that they drop off after gorging themselves with blood, and probably seek a retreat at the bottom of the sea, where they may hatch their young in safety. The digestive canal of _Æga_ dilates into a large bag, which becomes distended with a semi-solid mass of blood. This mass, when extracted and dried, is the "Peter's stone" of old Icelandic folklore, to which magical and medicinal virtues were attributed. The species _Æga spongiophila_, already mentioned, differs in its habits from all the other species of the genus, since it lives, not on fish, but in the interior of a sponge. [Illustration: _PLATE XXIX_ _Cymothoa oestrum_, AN ISOPOD PARASITE OF FISH (SLIGHTLY ENLARGED) _Sacculina carcini_ ATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB (REDUCED)]