Of all Crustacean parasites, however, perhaps the most remarkable in their structure and life-history are the Cirripedes of the order Rhizocephala. It is not uncommon on the British coasts to find specimens of the common Shore Crab or other Crabs which carry under the abdomen an oval fleshy body. This is the Rhizocephalan _Sacculina carcini_ (Plate XXIX.), and it would hardly be possible to guess, from its appearance or structure, that it was a Cirripede or a Crustacean at all. It is attached to the under-side of the Crab's abdomen by a short stalk, and in the middle of its opposite surface is a small opening which leads into a cavity separating the outer "mantle" from the body of the animal. Very often this mantle cavity will be found to be full of eggs enclosed in sausage-shaped packets. At the point where the short stalk enters the abdomen of the Crab, it gives off an immense system of fine branching roots, which penetrate throughout the body of the Crab, and even into its legs and other appendages. By means of these roots the _Sacculina_ absorbs nourishment from the body-fluids of its host. Like most Cirripedes, _Sacculina_ is hermaphrodite, and the body within the mantle cavity contains only the reproductive organs of the two sexes and a small nerve ganglion representing the whole of the nervous system. There is no mouth, no food-canal, and no trace of appendages. Another Rhizocephalan, _Peltogaster_, is not uncommonly found attached to the abdomen of Hermit Crabs. Although the nauplius larva of _Sacculina_ was described, and its resemblance to that of the Cirripedia pointed out, as long ago as 1836, by that acute observer, J. Vaughan Thompson, it is only recently that the full life-history has been made known by the researches of Professor Delage and Mr. Geoffrey Smith. The nauplius larva (Fig. 75, A) resembles that of the normal Cirripedes, especially in the shape of the dorsal shield, which is drawn out on either side in front into a pair of fronto-lateral horns. It has, however, no mouth, and the food-canal is quite absent. As in the normal Cirripedes, the nauplius is followed by a _cypris_ stage (Fig. 75, B), also mouthless, and it is in this form that the _Sacculina_ seeks the Crab on which it is to become parasitic. It would be almost impossible for the _cypris_ larva to settle on that part of the Crab where the adult _Sacculina_ is afterwards to appear, since the Crab usually has its abdomen closely pressed against the under-side of its thorax. The larva therefore attaches itself on some exposed part of the Crab, often on one of the legs, clinging to a hair by means of its antennules. It bores through the cuticle at the base of the hair, and the contents of its body pass into the interior of the Crab as a little mass of cells, the empty _cypris_ shell being cast off. This mass of cells, which constitutes the embryo _Sacculina_, is carried about by the blood-currents of the Crab