. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 212 ZOOLOGY the sand; these tubes are constructed of slime excreted from the body of the worm.^ The Leech. — The common leech or bloodsucker is a flattened seg- mented worm, inhabiting fresh-water ponds and rivers. The adult i? provided with two sucking disks, by means of which it fastens itself to objects. The mouth is on the lower surface close to the anterior disk. Locomotion is accomplished by swimming or by means of the suckers, somewhat after the manner of a measuring worm. They feed
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 212 ZOOLOGY the sand; these tubes are constructed of slime excreted from the body of the worm.^ The Leech. — The common leech or bloodsucker is a flattened seg- mented worm, inhabiting fresh-water ponds and rivers. The adult i? provided with two sucking disks, by means of which it fastens itself to objects. The mouth is on the lower surface close to the anterior disk. Locomotion is accomplished by swimming or by means of the suckers, somewhat after the manner of a measuring worm. They feed greedily and are often found gorged with blood, which they suck from the body of the victim. Discomfort, but no danger, attends the bite of the blood- sucker, so dreaded by the small boy. Unsegmented Worms. — Some worms are unsegmented; such are the flatworms and roundworms. A common leaflike form of flat worm may be found clinging to stones in our fresh water ponds or brooks. Most flat- worms are, however, parasites on other ani- mals. Of much in- terest to us is the life history of the flatworm infesting the liver of sheep, causing the dis- ease called liver rot, which causes annually a loss of several millions of dollars' worth of sheep. This worm is called the liver fluke because of its abode in the liver of the sheep. The developing eggs pass out from the liver into the intestine and thence outside of the body. If the egg happens to be deposited in water, it develops, otherwise it dies. The embryo is a little oval, ciliated creature, microscopic in size. This embryo swims about until it reaches a water snail. Here it lives as a parasite, loses its cilia, becomes larger, and gives rise to a number of little larvae called redice. The redise give rise to more larvae, some like themselves and others tadpole-shaped. The latter larvae leave the snail, swim. A flatworm (Yungia Aurantiaca), much magnified. From model in the American Museum of Natural History. * If the li\'ing
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