_Coronula diadema_ and _Tubicinella trachealis_ live embedded in the skin of whales, the shell of the first-named being of a highly complicated structure with hollow triangular compartments into which the mantle is drawn out. _Xenobalanus globicipitis_ lives attached to various Cetacea, and is remarkable for the rudimentary condition of its skeleton, the six plates of which form a mere disc of attachment from which the greatly elongated naked body rises, resembling one of the naked Stalked Barnacles. =Fam. 4 Tetrameridae.=—In this family only four skeletal plates are present (Fig. 63, C). This family is chiefly confined to tropical seas or those of the Southern hemisphere. The chief genera are _Tetraclita_ and _Pyrgoma_, found in British seas. =Sub-Order 3. Acrothoracica.= Gruvel includes in this sub-order four genera (_Alcippe_, _Cryptophialus_, _Kochlorine_, and _Lithoglyptes_) the species of which live in cavities excavated in the shells of molluscs or in the hard parts of corals. [Illustration: FIG. 64.—_Alcippe lampas._ =A=, ♀, × about 10, seen from the right side, with part of the right half of the animal removed; =B=, dwarf male, × about 30. _A.M_, adductor muscle; _An_, antenna; _C_, 1st pair of cirri; _Cr_, posterior thoracic appendages; _E_, eye; _G_, testis; _M.C_, mantle-cavity; _O_, ovary; _P_, penis; _T_, penultimate thoracic segment; _V._ vesicula seminalis. (After Darwin.) ] Darwin discovered and described _Cryptophialus minutus_, and placed it in a sub-order Abdominalia, believing that it was distinguished from all the foregoing Cirripedes by the presence of a well-developed abdomen. Since the discovery of other allied genera, it has been decided that the abdomen is equally reduced in these forms, and that the terminal appendages do not belong to this region, but to the thorax. The sexes are separate. The body of the female (Fig. 64, A) is enclosed in a chitinous mantle, armed with teeth by which the excavation is effected, and is attached to the cavity in the host by means of a horny disc. Upon this disc the dwarf males (B) are found. _Alcippe lampas_ inhabits holes on the inner surface of dead _Fusus_ and _Buccinum_ shells; _Cryptophialus minutus_ the shells of _Concholepas peruviana_; _C. striatus_ [68] the plates of _Chiton_; _Kochlorine hamata_ the shells of _Haliotis_; and _Lithoglyptes varians_ shells and corals from the Indian Ocean. =Sub-Order 4. Ascothoracica.= These are small hermaphrodite animals completely enveloped in a soft mantle, which live attached to and partly buried in various organisms, such as the branching Black Corals (_Gerardia_). They retain the thoracic appendages in a modified state, and the body is segmented into a number of somites, the last of which probably represents an abdomen.