Perhaps the most interesting feature of the genus is the remarkable variation in the sexual constitution of some of the species. The great majority of the Pedunculata and all the Operculata are hermaphrodites, which habitually cross-fertilise one another, and this they are well fitted to do, since they all live gregariously and are provided with a long exsertile penis for transferring the spermatozoa from one to the other. In _Pollicipes_, however, the individuals of which often live solitarily, it appears that self-fertilisation may occur. In _Scalpellum_ three different kinds of sexual constitution may occur: (1) According to Hoek in _S. balanoides_, taken by the _Challenger_, the individuals are ordinary cross-fertilising hermaphrodites. (2) In the great majority of species, including the common _S. vulgare_, as originally described by Darwin, and since confirmed by Hoek and Gruvel,[67] the individuals are hermaphrodite, but there are present affixed to the adult hermaphrodites, just inside the opening of the valves in a pocket of the mantle, a varying number of exceedingly minute males, called by Darwin “complemental males.” These tiny organisms are really little more than bags of spermatozoa, but they possess to varying degrees the ordinary organs of the adult in a reduced condition. The male of _S. peronii_ (Fig. 55, A) retains the shape and skeletal plates of the ordinary form, and differs chiefly in its reduced size; but the more common condition is exhibited by the male of _S. vulgare_ (Fig. 52, A), where the scutes are reduced to vestiges round the mantle-opening, and almost the whole of the body is occupied by the greatly developed generative organs. (3) In a few species, e.g. _S. velutinum_ and _S. ornatum_, the individuals are purely dioecious, being either females of the ordinary structure resembling the hermaphrodites of the other Lepadidae, or dwarfed males resembling closely the complemental males described above for _S. vulgare_. [Illustration: FIG. 56.—_Lithotrya dorsalis_, x 1. _B_, Basal calcareous cup; _C_, carina; _R_, rostrum; _S_, scutum; _T_, tergum. (After Darwin.) ] The nature and derivation of these various conditions will be discussed when the parallel cases found in _Ibla_ and among the Rhizocephala have been described. The remaining genus of the Polyaspidae, also characterised by the presence of numerous skeletal plates on the capitulum, is _Lithotrya_ (Fig. 56), which bores into rocks and shells, and is an inhabitant of the warm and tropical seas. The peduncle of the full-grown animal is completely imbedded in the rock or shell to which it is attached, and at the basal end of the peduncle is situated a cup composed of large irregular calcified pieces. This cup is, however, not formed until the animal has ceased to burrow. The excavation of the substratum is effected by means of a number of small rasping plates which cover the peduncle, the whole being set in motion by the peduncular muscles.