till it reaches the under-side of the intestine, where it becomes attached. It now begins to send out roots (Fig. 76), and as it grows the central mass travels backwards along the intestine of the Crab till it reaches the place where the adult parasite is to emerge. As the mass increases in size, and the organs of the _Sacculina_ become differentiated within it, its presence causes the living tissues between it and the external cuticle to degenerate, so that when the Crab moults an opening is left through which the body of the parasite protrudes. Owing, no doubt, to the drain on its system due to the presence of the _Sacculina_, the Crab ceases to grow, and it does not moult again as long as the parasite remains alive. [Illustration: FIG. 75--FREE-SWIMMING STAGES OF _Sacculina carcini_. MUCH ENLARGED. (After Delage.) A, Nauplius; B, cypris stage.] [Illustration: FIG. 76--EARLY STAGE OF _Sacculina_ WITHIN THE BODY OF A CRAB. (After G. Smith.) _i_, Intestine of the Crab; _s_, body of the _Sacculina_, which afterwards emerges on the under-surface of the Crab's abdomen; _r_, roots of the _Sacculina_.] In addition to this arrest of growth, _Sacculina_ produces in its hosts other changes, which affect chiefly the reproductive organs and the structures associated therewith. Crabs of either sex infected with _Sacculina_ are incapable of breeding; the genital gland (ovary or testis) is found on dissection to be shrivelled up, and the external characters indicative of sex become strangely modified. The changes have been most fully studied in the case of a kind of Spider Crab common at Naples--_Inachus mauritanicus_. In this species it is found that females infected with _Sacculina_ show no conspicuous external modification, except that the abdominal appendages, which in the normal females serve for the attachment of the eggs, are greatly reduced in size. Infected males, however, may assume to a greater or less degree the characters proper to the female sex. Some males show little change, except that the chelipeds remain small and flattened, as in the females and non-breeding males. Other specimens have, in addition, the abdomen much broader than in normal males, and sometimes as broad as in the females. Finally, some males develop on the abdomen, in addition to the rod-like appendages on the first and second somites, characteristic of the male sex, two-branched appendages on the next three somites, as in the females; these individuals are, in fact, so completely intermediate in character between the two sexes that it is only by dissection that it is possible to recognize them as modified males.