These remarkable animals are Cirripedes which have taken to living parasitically on various kinds of Crustacea; the majority infest species of Decapoda, e.g. _Peltogaster_ on Hermit-Crabs, _Sacculina_ on a number of Brachyura, _Sylon_ on Shrimps, _Lernaeodiscus_ on _Galathea_; but one genus, _Duplorbis_, has been found in the marsupium of the Isopod _Calathura brachiata_ from Greenland. Most of the species are solitary, but a few, e.g. _Peltogaster sulcatus_, are social. In the adult state the body consists of two portions: a soft bag-like structure, external to the host, carrying the reproductive, nervous, and muscular organs, and attached to some part of the host’s abdomen by means of a chitinous ring; and a system of branching roots inside the host’s body, which spring from the ring of attachment and supply the external body with nourishment. [Illustration: FIG. 66.—Nearly median longitudinal section (diagrammatic) of _Peltogaster_. _gn_, Brain; _m_, mantle; _mc_, mantle-cavity; _mes_, mesentery; _op_, mantle-opening; _ov_, ovary; _ovd_, oviduct; _ring_, ring of attachment; _t_, testis; _vd_, _vas_ deferens. ] [Illustration: FIG. 67.—Diagrammatic median longitudinal section through a normal Cirripede, _gn_, Brain; _op_, mantle-opening; _ovd_, oviduct; _vd_, vas deferens. ] The structure of the external bag-like portion is very simple, and varies only in details, chiefly of symmetry, in the different genera. In _Peltogaster_, which preserves the simplest symmetrical arrangement of the organs, a diagrammatic section through the long axis of the body (Fig. 66) shows that it consists of a muscular mantle (_m_) surrounding a visceral mass, and enclosing a mantle-cavity (_mc_) or brood-pouch, which stretches everywhere between mantle and visceral mass, except along the surface by which the parasite is attached to its host, where a mesentery (_mes_) is formed. The ring of attachment is situated in the middle of this mesentery; the mantle-cavity, which is completely lined externally and internally with chitin, opens anteriorly by means of a circular aperture (_op_) guarded by a sphincter muscle. The visceral mass is composed chiefly of the two ovaries (_ov_), which open on either side of the mesentery by means of a pair of oviducts (_ovd_); the paired testes (_t_) are small tubes lying posteriorly in the mesentery, and the nervous ganglion (_gn_) lies in the mesentery between oviducts and mantle-opening. A comparison with the condition of a normal Cirripede (Fig. 67) shows us that the mesenterial surface of the parasite by which it is fixed corresponds to the dorsal surface of an ordinary Pedunculate Cirripede, and that the ring of attachment corresponds with the stalk or peduncle of a _Lepas_. The root-system passes out through the ring of attachment into the body of the host, and ramifies round the organs of the crab; the roots are covered externally with a thin chitinous