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 (Fig. 76). 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 (Fig. 78). The nephridia absorb waste water from the liquid in the celom, or body cavity surrounding the food tube, and convey it to the outside.

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 dioxid passing out; hence it is constantly breathing through all parts of its skin. It needs no lungs nor special respiratory organs of any kind.

—Sperm (sp) and egg glands (es) of earthworm.

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 ova or egg glands consist of two pairs of small pores found on the ventral surface of the fourteenth and fifteenth segments in most species (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 sperm glands are in front of the ovaries (Fig. 77), but the sperm ducts are longer than the oviducts, and open behind them (Figs. 77, 78). The worms exchange sperm cells, but not