238 ENTO-PARASITES AND HYGIENE. 



into the organism of the definite host, grow up to be 

 sexual animals. This fact is especially interesting since 

 it shows that the young parasites must acquire for the 

 ■period of their free life a sum of contrivances which they 

 actually need for it, especially organs of locomotion and 

 of sense. Other larvse, waiting quietly in the interior of 

 their host, do not need such arrangements, and they de- 

 velop directly into the final form, while the larvae with 

 a period of free life provide themselves with organs 

 which they later throw off or lose again. Thus these 

 young trematods possess a very movable tail, which helps 

 in the locomotion. They have eyes, etc., all organs 

 which later on disappear again. These forms were at 

 first called " cercaria," because scientists considered them 

 as independent species; later it was discovered that they 

 were only undeveloped forms of other well-known trem- 

 atods, but they kept this name. These cercariae have a 

 very limited active life, and in order to prolong it, and 

 the possibility of reaching a host, they encyst, i. e., they 

 form a little capsule around themselves and wait until 

 they are introduced passively into a new host, usually by 

 means of its food. 



Thus the cercaria of Distomum hepaticum, the fluke 

 of the Ruminantia, encyst on plants, grass and leaves, 

 with which they are eaten by the beasts. Others select 

 animals, penetrate into their bodies and become thus 

 parasites again before having even reached their fi.nal 

 host. 



This second intermediate state is always temporary. 

 The parasites do here not reach any higher state of de- 

 velopment, they must wait until their host is devoured by 

 another animal. In such a case it may happen that they 

 receive companions in the course of time. 



Of two cercarise of the same sporocyst which went 

 out and found their intermediate host, one succeeded 

 perhaps soon in reaching the final host and producing 



176 



