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 sandworm is obtainable, a laboratory period may be devoted to 

 its activities. See Hunter and Valentine, Manual^ page 133. 



