=Nervous System.=--Between the ventral blood vessels is a _nerve cord_ composed of two strands (see Fig. 75). There is a slight swelling, or _ganglion_, on each strand, in each segment (Fig. 75). The strands separate near the front end of the worm, and a branch goes up each side of the gullet and enters the two pear-shaped _cerebral ganglia_, or “brain” (Fig. 75). [Illustration: FIG. 75.--GANGLIA NEAR MOUTH and part of nerve chain of earthworm.] =Food.=--The earthworm eats earth containing organic matter, the inorganic part passing through the vent in the form of circular casts found in the morning at the top of the earthworm’s hole. What else does it eat? The earth worm needs no teeth, as it excretes through the mouth an _alkaline fluid_ which softens and partly digests the food before it is eaten. When this fluid is poured out upon a green leaf, the leaf at once turns brown. The starch in the leaf is also acted upon. The snout aids in pushing the food into the mouth. =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 (Fig. 76). There is a pair on the floor of each segment (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. [Illustration: FIG. 76.--TWO PAIRS OF NEPHRIDIA.] =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. [Illustration: FIG. 77.--Sperm (_sp_) and egg glands (_es_) of earthworm.]