crab’s epithelium to degenerate, so that when the crab moults, a little hole is left in this region of the same size as the body of the _Sacculina_, owing to the failure of the epithelium to form chitin here; and thus the little parasite is pushed through this hole and comes to the exterior as the adolescent “_Sacculina_ externa.” From this point onwards the crab, being inhibited in its growth through the action of the parasite, never moults again; so that the _Sacculina_ occupies a safe position protruding from the crab’s abdomen, which laps over it and protects it. The remarkable features of this development are, firstly, the difficulty of understanding how the developing embryo is directed in its complicated wanderings so as always to reach the same spot where it is destined to come to the exterior; and, secondly, the loss after the Cypris stage of all the organs and the resumption of an embryonic undifferentiated state from which the adult is newly evolved. A certain parallel to this history is found in that of the Monstrillidae, described on pp. 64–66. [Illustration: FIG. 69.—The mid-gut of _Inachus mauritanicus_ with a young _Sacculina_ overlying it, × 2. _c.t_, “Central tumour” of the parasite; _d.i_, _d.s_, inferior and superior diverticula of alimentary canal of host; _n_, “nucleus,” or body-rudiment of _Sacculina_; _r_, its roots; _x_, definitive position of the parasite. ] [Illustration: FIG. 70.—Later stage in the development of the “_Sacculina_ interna,” × 2. _b_, Body of _Sacculina_; _c.t_, “central tumour”; _d.i_, _d.s_, inferior and superior diverticula of alimentary canal of host; _o_, opening of perivisceral cavity of _Sacculina_; _r_, its roots. ] [Illustration: FIG. 71.—Fourteen Cypris larvae fixed round the mantle-opening (_o_) of a young _Sacculina_ externa, × 20. ] The Rhizocephala are hermaphrodite with the possible exception of _Sylon_, which appears to be female and perhaps parthenogenetic, no male having been seen; but unlike most other hermaphrodite Cirripedes, they reproduce by a continual round of self-fertilisation. This is the more remarkable in that the vestiges of what appears to be a male sex are still found in _Sacculina_ and _Peltogaster_; certain of the Cypris larvae in these genera, instead of fixing on and inoculating other crabs, become attached round the mantle-openings of young parasites of the same species as themselves, which have recently attained to the exterior of their hosts (Fig. 71). These larvae, which remind us of the complemental males in _Scalpellum_, etc., never produce spermatozoa, but rapidly degenerate where they are fixed, and appear never to play any rôle in the reproduction of their species. The nature of this remarkable phenomenon, together with the sexual condition of the Cirripedes in general, will be discussed in the next section.