The Rhizocephala constitute a peculiar group of Crustaceans, the adults of which are found attached parasitically to the abdomen of Crabs and Hermit-crabs. The body (fig. 141, B) is sac-like, and non-segmented, and consists of a muscular mantle in which no skeletal structures are developed, its only aperture being reproductive and closed by a sphincter. There are no limbs, sense-organs, or alimentary canal, but there are well-developed reproductive organs, each individual, according to Giard, being hermaphrodite. The sac-like body is kept in connection with its host by means of branched, root-like processes of attachment (fig. 141, B), which sink deeply into the tissues of the latter. These processes appear to correspond with the "cement-ducts" of the Cirripedes, and to be, therefore, really the homologues of the antennae. By their means, the parasite draws nutriment from its host; and as similar hoilow nutritive processes are developed on the "peduncle" of certain Barnacles (Anelasma squalicola), there are grounds for accepting Kossmann's view that the Rhizocephala are really to be regarded as a degraded group of the Cirripedia.
* The name of "Nauplius" was given by O. F. Muller to the unseg-mented ovate larva of the lower Crustacea, with a median frontal eye, but without a true carapace ; and this name may be conveniently employed to designate all the larval forms which agree in these characters.
Fig. 141. - Morphology of Rhizocephala. A, First larval form of Sacculina purpurea, greatly enlarged. B, Young of Peltogaster sociahs attached to the abdomen of a Hermit-crab; at a the root-like processes of attachment of one individual are shown. C, body of Sacculina carcini, of the natural size, the roots of attachment not shown. (A and B are after Fritz Muller.)
The embryos of the Rhizocephala (fig. 141, A) are at first "naupliiform," with an ovate unsegmented body, an unpaired median eye, and a dorsal shield or carapace. The abdomen terminates in a movable caudal fork, and there is neither mouth nor alimentary canal. In their second stage (as so-called "pupae"), the young of the Rhizocephala are enclosed in a bivalve shell, the foremost pair of limbs constitute peculiar organs of adhesion ("prehensile antennae" of Darwin), the two following pairs of limbs are cast off, and six pairs of powerful biramose natatory feet are formed on the thorax. There is still no mouth. The "pupae" now attach themselves to the abdomen of Crabs, Porcellanae, and Hermit-crabs; they remain astomatous; "they lose all their limbs completely, and appear as sausage-like, sack-shaped, or discoidal excrescences of their host, filled with ova; from the point of attachment closed tubes, ramified like roots, sink into the interior of the host, twisting round its intestine, or becoming diffused amongst the sack-like tubes of its liver. The only manifestations of life which persist in these non plus ultras in the series of retro-gressively metamorphosed Crustacea are powerful contractions of the roots, and an alternate expansion and contraction of the body, in consequence of which water flows into the broodcavity, and is again expelled through a wide orifice." (Fritz Muller).