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.

PLATE XXIX

Cymothoa œstrum, AN ISOPOD PARASITE OF FISH (SLIGHTLY ENLARGED)

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Sacculina carcini ATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB (REDUCED)