142 ANIMAL LIFE AND HUMAN PROGRESS 



large numbers determined to feed a calf with eggs passed by 

 a patient known to be infected by this hookless form. The 

 calf, when killed later, was found to be infested with cystic 

 worms which had a somewhat different appearance from that 

 of the cystic stage of the armed tapeworm from the muscles 

 of the pig. Some of the cysts were swallowed by a volunteer 

 in whom they developed into tapeworms of the hookless 

 variety. These experiments, which apparently proved that 

 man harbours, in his alimentary canal, not one but at least 

 two species of tapeworm, developing respectively in the pig 

 and in cattle, were at first received with incredulity, for cystic 

 worms had never been recorded in cattle killed at the abattoirs. 

 When specially looked for, however, they proved to be of quite 

 common occurrence. 



These studies in experimental zoology revealed the source 

 of the " infective " stage of three of the important tapeworms 

 of man and led to the routine meat inspections by our public 

 health authorities, which are now so efficiently carried out 

 that tapeworm infection is relatively rare in this country. 

 They showed too that certain parasitic worms at least 

 require a second and different species of host in which to pass 

 a portion of the life-cycle and that each species has its own 

 special intermediary. Thus, the armed tapeworm can only 

 attain its cystic infective stage in the pig, while the hookless 

 form can similarly develop only in cattle. 



The importance of experimentally determining the special 

 intermediary host for each species is illustrated by the migra- 

 tions of the " broad " tapeworm, 1 which is a common parasite 

 in man in the countries bordering the Baltic and in Switzer- 

 land. This species is practically unknown save where fresh- 

 water fish form an important item of diet. In 1881 Braun 

 found in the flesh of perch, pike and allied fish a larval stage 

 which when swallowed by volunteers eventually developed 

 into mature specimens of the " broad " tapeworm. Fresh- 

 water fish, then, were undoubtedly the essential intermediaries 

 of the " infective " stage. Repeated experiments, however, 

 failed to produce the developmental stages by submitting 



1 Bothriocephalus latus, sometimes termed the Irish tapeworm. 



