INVERTEBRATA. 



373 



9. Development of the Tape-worm. When measly 

 pork is eaten by human beings the meat is digested and 

 the cysticercus (or several of them) is set free. Thorough 

 cooking kills the cysticerci and prevents infection. In the 

 duodenum of the new host the scolex or head of the cysticer- 

 cus is everted, and attaches itself to the mucous membrane by 

 its suckers and hooks. The bladder is digested and disap- 

 pears, and the neck begins to grow and develop proglottids 

 until the adult stage already described is fully developed. 

 Thus the life-cycle is completed, and we see that it requires 

 two distinct hosts, both in this case belonging to the 

 Mammalia. 



10. Interpretation of the Structure of Taenia. At 



first sight it seems obvious that the head or scolex of the 

 tape-worm is the anterior end, and if this is correct it 

 follows that the formation of the proglottids is not com- 

 parable to that of the segment in an Annelid such as the 

 earthworm or Nereis. In the latter the new segment is 

 formed between the last segment and the posterior ex- 

 tremity, in the tape-worm the new segment is formed 

 between the first and the head. The proglottids might be 

 on this view regarded as buds, comparable to the zooids 

 which are budded off in succession from certain Annelids, 

 each bud arising from the anterior asexual region. 



There is, however, some reason for the view that the so- 

 called head of Taenia really corresponds to the anal 

 segment of an Annelid and is not anterior but posterior. 

 Assuming that the six spines of the embryo are developed 

 at the anterior end, it is certain that in the larva of 

 Dipylidium caninum, a tape-worm of the dog, and in other 

 species, the scolex is developed at the end opposite to the 

 hooks, and is therefore posterior. On this view the 

 proglottids of the tape- worm would be formed much in the 

 same way as the new segments in the Annelid, and the 

 structure of the tape-worm would be a true instance of 

 metameric segmentation. 



Another circumstance in favour of this interpretation is 

 that if we compare the structure of a proglottis of Taenia 

 with that of Dislomum hepaticum, the Liver-fluke, the 



