On the basal joint of each antenna it is easy to see a small conical eminence with an opening on the inner side of its summit (fig. 18). The aperture (_x_) leads by a short canal into a spacious sac, with extremely delicate walls (_s_), which is lodged in the front part of the head, in front of and below the cardiac division of the stomach (_cs_). Beneath this, in a sort of recess, which corresponds with the epistoma, and with the base of the antenna, there is a discoidal body of a dull green colour, in shape somewhat like one of the fruits of the mallow, which is known as the _green gland_ (_gg_) The sac narrows below like a wide funnel, and the edges of its small end are continuous with the walls of the green gland; they surround an aperture which leads into the interior of the latter structure, and conveys its products into the sac, whence they are excreted by the opening in the antennary papilla. The green gland is said to contain a substance termed _guanin_ (so named because it is found in the _guano_ which is the accumulated {83} excrement of birds), a nitrogenous body analogous in some respects to uric acid, but less highly oxidated; and if this be the case, there can be little doubt that the green gland represents the kidney, and its secretion {84} the urinary fluid, while the sac is a sort of urinary bladder. [Illustration: FIG. 18.—_Astacus fluviatilis._—A, the anterior part of the body, with the dorsal portion of the carapace removed to show the position of the green glands; B, the same, with the left side of the carapace removed; C, the green gland removed from the body (all × 2). _ag_, left anterior gastric muscle; _c_, circumœsophageal commissures; _cs_, cardiac portion of stomach; _gg_, green gland, exposed in A on the left side by the removal of its sac; _ima_, intermaxillary or cephalic apodeme; _œs_, œsophagus seen in transverse section in A, the stomach being removed; _s_, sac of green gland; _x_, bristle passed from the aperture in the basal joint of the antenna into the sac.] * * * * *