SOME INHABITANTS OF MAN 143 



these species of fish, to infection with the eggs of the adult 

 parasite. Last year, however, Janicki showed that a second 

 intermediary is essential. After hatching from the eggs, the 

 tapeworm young are pursued and swallowed by a species of 

 Cyclops. In the body of this minute crustacean the so- 

 called " water-flea " they undergo a preliminary develop- 

 ment. If the infected Cyclops is swallowed by a perch, for 

 example, the parasite is set free and migrates into the 

 muscles of the fish and there, after further growth, attains 

 the infective stage ; remaining in this situation until the fish 

 is in turn eaten by a human being. 



Nearly related to the tapeworms are the flukes or Trema- 

 toda. Perhaps the best-known example of the group is the 

 liver-fluke, Fasciola hepatica, which infests sheep and cattle 

 and causes heavy losses in all meat-producing countries. In 

 man, allied species of flukes inhabit the liver, the lungs and 

 the blood-vessels, especially in tropical lands, where they cause 

 grave and incurable diseases. There is no successful method 

 of attacking the adult flukes and there seems little probability 

 that they will be eradicated by drug administration, as they 

 reside in situations inaccessible to medication. As in the 

 case of the tapeworms, however, zoological studies have 

 enabled us to attack these parasites successfully during their 

 migrations outside the body. 



Our knowledge of these migrations dates from a remarkable 

 series of observations made by the Danish zoologist van 

 Steenstrup. Several groups of " asexual flukes " had been 

 recognised by early workers in invertebrate zoology. These 

 had been placed in isolated " genera," to which the names 

 Redia, Cercaria etc., were given. Thus the so-called genus 

 Cercaria comprised larval Trematoda which enjoy the power 

 of free locomotion in water and are provided with a tail 

 wherewith to propel themselves. It was recognised that 

 this form was not a permanent one, but the changes which it 

 underwent were unknown. Steenstrup showed that these 

 free -swimming " cercariae " in some instances attacked 



