[Illustration: FIG. 57.—_Conchoderma virgata_, × 1. _C_, Carina; _S_, scutum; _T_, tergum. (After Darwin.) ] =Fam. 2. Pentaspidae.=—In this family are placed a number of genera, and among them the common _Lepas_, the species of which possess typically five skeletal plates, viz., a carina and a pair of scuta and of terga, the peduncle being naked. These forms are a later development of Cirripede evolution, and did not come into existence till Tertiary times. Some of them, e.g. _Oxynaspis_, live at considerable depths attached to corals, etc., but large numbers float on the surface of the sea, fixed often on logs and wreckage of various kinds. _Dichelaspis_ is found attached to the shells of large Crustacea. _Conchoderma_ is an interesting genus, the species of which live affixed to various floating objects, the keels of ships, etc.; the mantle is often brilliantly coloured, as in _C. virgata_, and the skeletal plates are reduced to the merest vestiges, leaving the greater part of the body fleshy. [Illustration: FIG. 58.—_Ibla cumingii_, ♀, × 1. _S_, Scutum; _T_, tergum. (After Darwin.) ] [Illustration: FIG. 59.—_Ibla cumingii_, dwarf male, × 32. _A_, Antennae; _B_, part of male imbedded in the female, to which the torn membrane _M_ belongs; _E_, eye; _Th_, thoracic appendages or cirri. (After Darwin.) ] =Fam. 3. Tetraspidae.=—This family includes the single genus _Ibla_ (Fig. 58), which possesses only four skeletal plates, a pair of terga and of scuta, coloured blue, while the peduncle is covered with brown spines. There are only two very similar species known, _I. cumingii_, which is found attached to the peduncle of _Pollicipes mitella_, and _I. quadrivalvis_, living on masses of the Siphonophore _Galeolaria decumbens_. These two species are quite different in the partition of the sexes. In _I. cumingii_ the large individuals of normal structure are females, inside the mantle-cavities of which are attached dwarf males of the form shown in Fig. 59. These organisms have the peduncle buried completely in the substance of the female’s mantle, inside which they live; they exhibit a degenerate structure, but still retain two pairs of cirri. The large individuals of _I. quadrivalvis_, on the other hand, are hermaphrodites, but they harbour within their mantles minute complemental males similar to those of _I. cumingii_, though they are rather larger. [Illustration: FIG. 60.—Diagram of the shell of an Operculate Cirripede. _a_ “Ala,” or overlapped portion of a “compartment”; _B_, basis; _C_, carina; _C.L_, carino-lateral; _L_, lateral; _R_, rostrum; _r_, “radius,” or overlapping portion of a compartment; _R.L_, rostro-lateral. (After Darwin.) ]