146 FIRST LESSONS IN ZOOLOGY 



sucking cause very littk: pain, and in olden days physi- 

 cians used the leeciies when they wanted to "bleed" 

 a person. A common European species much used 

 for this purpose is known as the "medicinal leech.'' 

 Most of the leeches lay their eggs in small packets or 

 cocoons. These cocoons are dropped in soil on the 

 banks of a pond or stream so that the young may have a 

 moist but not too \\'et environment. The young issue 

 from the eggs in four or five weeks, but they grow very 

 slowly and it is several 3'ears before they attain their full 

 size. Leeches are long-lived animals, some being said 

 to live for twenty }'ears. 



Vinegar-eels, hairworms, and trichinae. — The group 

 of round\\'orms, so called from their slender, smooth, 

 cylindrical bodies, contains some interesting animals. 

 Familiar examples are the vinegar-eels (fig. 108) which 

 can be found in mouldy vinegar, and the hairworms or 

 horse-hair snakes which are often seen in fresh-water 

 pools after a rain. Some people believe these worms to 

 be horsehairs which have turned into animals, and others 

 believe that they come down with the rain. They have 

 in reality come from the bodies of insects in which they 

 pass their }'oung or larval stages as parasites. The hair- 

 worms all live as parasites during their larval stages, and 

 as free independent animals in the adult. A parasite is 

 an animal wliich lives for part or all of its life in or on 

 the body of another animal called the host, and which 

 feeds on the blood or other tissues of this host. Some ot 

 the hairworms require two distinct hosts for the com- 

 pletion of their larval life, living for a while in the body 

 of one, and later in the bod}- of another. The first host 

 is usual!)' a kind of insect which is eaten b}- the second. 

 The eggs are deposited by the free adult female in 

 slender stri)igs twisted around the stems of water-plants. 

 The }oung hairworm on hatching sinks to the bottom of 



