{214}[Illustration: FIG. 112.—Apsilidae: A, _Apsilus lentiformis_, ♀, dorsal view (after Metschnikoff); the square brain is seen with nerves to the lateral antennae; B, larva of _A. lentiformis_ (?), showing the paired eyes and ciliated cupped foot; C, adult of _Atrochus appendiculatus_, ♂ (after Wierzejski). _al_, Lateral antennae; _am_, median antenna (just in front is seen the renal commissure); _an_, anus; _br_, brain, below which the paired eyes are seen; _c_, cloaca; _em_, embryo; _em'_, _em'_, _em'''_, three successive stages of embryos in the uterus of C; _k_, kidney. The coarser muscles are striated.] This bladder may reach when expanded one-third the diameter of the whole animal, and contract as often as three times per minute; so that in a period of nine minutes a bulk of water equal to that of the animal must have diffused through the body-wall, to be removed by the kidneys. It is obvious that while the function of the kidneys is primitively excretory, the passage of the water through the body must bring in the oxygen dissolved in the external {215}medium, and carry off the carbonic acid formed in the tissues, and so fulfil the act of respiration. This mechanism is physiologically comparable with that of the contractile vacuole of fresh-water Protozoa. In a few genera (_Conochilus_, _Lacinularia_, _Pterodina_) the kidneys open separately after a slight dilatation into the cloaca. NERVOUS SYSTEM.—The nervous centre of the Rotifera is the _brain_ (Fig. 112, C, _br_), a ganglion lying dorsal to the pharynx; and when this is short it may be immediately below the surface of the disc (_Microcodon_). In Bdelloida a second ganglion is present below the pharynx, and is connected with the former by lateral cords which contain ganglion cells. From the brain, nerves are given off to the disc, to the muscles, and to the integument of the body, as well as to the sense organs. The largest nerves are two given off from the sides of the brain, each of which divides into a lateral and a ventral trunk, which run nearly the whole length of the animal. The brain of several Notommatidae has a curious appendage, white by reflected light and very opaque; it is a sac full of chalky mineral matter, which dissolves readily in dilute acids.