. The animans and man; an elementary textbook of zoology and human physiology. Internal parasites.—Inside the body of most animalslive various parasites belonging to the great branch of tapeworm and the deadly trichina (fig. 206; for ac-count see p. 147) are conspicuous examples of these. Thetapeworm (fig. 207) has the form of a narrow ribbon, per-haps several yards long, attached at one end to the wall ofthe intestine, while the remainder hangs freely in the body is composed of segments or serially arranged parts,of which there are about 850 altogether. It has nomouth o
. The animans and man; an elementary textbook of zoology and human physiology. Internal parasites.—Inside the body of most animalslive various parasites belonging to the great branch of tapeworm and the deadly trichina (fig. 206; for ac-count see p. 147) are conspicuous examples of these. Thetapeworm (fig. 207) has the form of a narrow ribbon, per-haps several yards long, attached at one end to the wall ofthe intestine, while the remainder hangs freely in the body is composed of segments or serially arranged parts,of which there are about 850 altogether. It has nomouth or stomach. It feeds simply by absorbing into itsbody, through the skin, the nutritious food already partly 414 THE ANIMALS AND MAN digested in the intestine of its host. It has no eyes or otherspecial sense-organs, nor any organ of locomotion. Thusits body is very degenerate. The life-history of thetapeworm is interesting, because it lives in two hostsduring its life. The eggs of this parasite pass fromthe intestine with the excreta, and to develop must /•> i I .1 !. FIG. 206. Trichina spiralis, encysted in muscle of a pig. (Greatly en-larged.) be taken into the body of some other animal. In thecase of one of several species infesting man this second hostis the pig. In the alimentary canal of the pig the youngtapeworm develops, to bore its way later through the wallsof the canal and become imbedded in the muscles. There it ANIMAL PARASITES AND DEGENERATION 415 lies until the diseased flesh containing it is eaten (withoutbeing perfectly cooked), and thus it finds its way into thealimentary canal and thence into the intestine ofman. It now continues to develop until it becomesfull grown. Many animals are infested by minute parasites belongingto the Protozoa or one-celled animals. The class Sporozoaof the Protozoa is composed almost exclusively of parasiticspecies living in the blood, liver, alimentary canal, and
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