Not only the terminal joints of the palpi, but also the next, and sometimes next two joints, are modified in shape, with the development of the palpal organ, Fig. 48. The shape of these organs is very constant in the same species of spider, and thus becomes one of the most convenient marks of such a group. THE EPIGYNUM. When the female spider is nearly full grown, there appears a hard, swollen place just in front of the opening of the ovaries, Fig. 1; and, after the last moult, the epigynum is uncovered at this place. The epigynum, Fig. 49, consists of two spermathecæ, E, which connect by two little tubes, H, H, with the oviduct near its mouth, and by two larger tubes, D, with the outside of the spider. The mouths of these larger tubes are often surrounded by various hard appendages to support and guide the palpal organs when the latter are thrust into them. These parts, like the palpal organs, furnish convenient marks for distinguishing species. The spermathecæ, E, vary but little in shape in different spiders; but the tubes H and D are often lengthened, and twisted into shapes nearly as complicated as those of the palpal organs. Fig. 50 is the epigynum of a small _Theridion_, where the arrangement of parts can be very distinctly seen. E, E, are the spermathecæ; H, H, the tubes opening into the oviduct; and D, D, the tubes opening outward. Fig. 51 is the epigynum of another closely allied species, where the tubes D, D, are very much elongated and twisted up, corresponding to the long discharge-tube of the palpal organ of the male of the same spider, Fig. 48. [Illustration: Fig. 49.] [Illustration: Fig. 50.] [Illustration: Fig. 51.] USE OF THE PALPAL ORGANS AND EPIGYNUM. When the reproductive cells of the male spider are mature, he discharges the liquid containing them on a little web spun for the purpose; dips his palpal organs into it, and in a few moments takes up the whole, it is supposed, into the little sacs, Figs. 47, 48, inside the bulb; then he seeks the female, and inserts the palpal organs into her epigynum. The soft part at the base of the organ swells up, and presses in the discharge-tube, and probably forces out the contents of the bulb into the spermathecæ, E, E, from which it escapes, in course of time, by the tubes, H, H, into the oviduct, and fertilizes the eggs about the time they are laid. [Illustration: Fig. 52.]