146 THE ANIMALS AND MAN 



live mostly on the blood of other animals. The common 

 leech fastens itself upon its victim by means of its suckers, 

 then cuts the skin, fastens its oral sucker over the wound, 

 and pumps away until it has completely gorged itself with 

 blood, distending enormously its elastic body, when it 

 loosens its hold and drops off. Its biting and sucking 

 cause very little pain, and in olden days physicians used 

 the leeches 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 wet environment. 

 The young issue from the eggs in four or five weeks, but 

 they grow very slowly and it is several years before they 

 attain their full size. Leeches are long-lived animals, some 

 being said to live for twenty years. 



The group of roundworms, so called from their slender, 

 smooth, cylindrical bodies, contains some interesting ani- 

 mals. Familiar examples are the vinegar-eels, 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 young or 

 larval stages as parasites. The hairworms all live as para- 

 sites during their larval stages, and as free independent ani- 

 mals in the adult. A parasite is an animal which 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 of the hairworms require two distinct 

 hosts for the completion of their larval life, living for a while 

 in the body of one, and later in the body of another. The 

 first host is usually a kind of insect which is eaten by the 



