=Kidneys.=—Since oxidation is occurring in its tissues, and impurities are forming, there must be some way of _removing impurities from the tissues_. The earthworm does not possess one-pair organs like the kidneys of higher animals to serve this purpose, but it has numerous pairs of small tubular organs called _nephridia_ which serve the purpose. Each one is simply a tube with several coils. There is a pair on the floor of each segment. Each nephridium has an inner open end within the body cavity, and its outer end opens by a pore on the surface between the setæ. The nephridia absorb waste from the liquid in the _celom_, or body cavity surrounding the food tube, and convey it to the outside. [Illustration: FIG. 76.—TWO PAIRS OF NEPHRIDIA in a worm (diagram). ] =Respiration.=—The skin of the earthworm is moist, and the blood capillaries approach so near to the surface of the body that the oxygen is constantly passing in from the air, and carbon dioxide passing out; hence it is constantly breathing through all parts of its skin. _It needs no lungs_ nor special respiratory organs of any kind. [Illustration: FIG. 77.—Sperm (_sp_) and egg glands (_es_) of worm. ] =Reproduction.=—When one individual animal produces both sperm cells and egg cells, it is said to be hermaphrodite. This is true of the earthworm. The egg cell is always fertilized, however, not by the sperm cells of the same worm, but by sperm cells formed by another worm. The openings of these _ovaries_ consist of two pairs of small pores found in most species on the ventral surface of the fourteenth segment (see Fig. 77). There are also two pairs of small _receptacles_ for temporarily holding the _foreign sperm cells_. One pair of the openings from these receptacles is found (with difficulty) in the wrinkle behind the ninth segment (Fig. 77), and the other pair behind the tenth segment. The _spermaries_ are in front of the ovaries (Fig. 77), but the _sperm ducts_ are longer than the _oviducts_, and open behind them on the fifteenth segment (Figs. 77, 78). The worms exchange sperm cells, but not egg cells. The reproductive girdle, or _clitellum_, already spoken of, forms the case which is to hold the eggs (see Fig. 71). When the sperm cells have been exchanged, and the ova are ready for fertilization, the worm draws itself backward from the collar-like case or clitellum so that this slips over the head. As it passes the fourteenth segment, it collects the ova, and as it passes the ninth and tenth segments, it collects the sperm cells previously received from another worm. The elastic, collar-like clitellum closes at the ends after it has slipped over the worm’s head, forming a _capsule_. The ova are _fertilized in this capsule_, and some of them hatch into worms in a few days. These devour the eggs which do not hatch. The eggs develop into complete but very small worms before escaping from the capsule. [Illustration: