Note the dorsal fold of the intestine, _typhlosole_, extending into the lumen. This fold gives a greater surface for digestion, and in it are a great many _hepatic_ or special _digestive cells_. The entire alimentary canal is lined with _epithelium_. Observe just beneath the alimentary canal the _ventral blood-vessel_, and still beneath this blood-vessel the _ventral nerve-cord_. There is a slight swelling on the nerve-cord in each segment of the body. These swellings are the _ganglia_. How many pairs of nerves are given off from each ganglion? Observe in each segment, posterior to the first three or four, the successive pairs of convoluted tubes, the _nephridia_, or organs of excretion. Each nephridium opens internally through a ciliated funnel, the _nephrostome_, within the body-cavity, while it opens externally by a small excretory pore between the setæ on the ventral surface of the segment behind that in which the nephridium chiefly lies. The function of the nephridia is to carry off waste matter from the fluid which fills the body-cavity. [Illustration: FIG. 26.--Dissection to show alimentary canal in section and nephridia of earthworm.] Trace the ventral nerve-cord forward to its connection with the cerebral ganglion. Note the throat nerve-ring or _circumœsophageal collar_ connecting the ventral cord with the brain. Make a drawing of the nervous system showing its relation to other organs. [Illustration: FIG. 28.--Cross-section of earthworm.] =Life-history and habits.=--The earthworm lives in soft moist soil which is rich in organic matter. Its food is taken into the mouth mixed with dirt and sand. As this mixture passes through the long alimentary canal the organic particles are taken up and digested. As we have already seen, there are in each worm two sets of reproductive glands, namely, male and female organs. Each earthworm produces both egg-cells and sperm-cells, but the sperm-cells of one worm are not used to fertilize the eggs of the individual producing them. When the eggs are ready to be discharged from the body, the clitellum becomes very much swollen and its glands begin an active secretion which hardens and forms a collar-like structure about the body of the worm. As this collar moves forward toward the anterior end of the body it collects the eggs and also the sperm-cells previously received from another worm, and finally slips off the head end of the animal. The entire structure with the contained eggs and sperm-cells as it passes off from the body becomes closed at both ends, thus forming a horny capsule which lies in the earth until the young worms emerge. Only a part of the eggs develop in each capsule, the rest being used as food for the growing young. The young earthworms, though of very small size, are fully formed before they leave the egg-capsule. Earthworms are more or less gregarious, large numbers often being found together.