374 ZOOLOGY. 



testes in the latter are posterior to the ovary, and there- 

 fore the side on which they are placed in the former, i.e. to- 

 wards the scolex, should also be posterior. In many Trema- 

 todes again, though not in Distomum, the chief organ 

 of attachment is posterior, and many of these animals are 

 in the habit of attaching themselves by the posterior 

 end and moving the anterior mouth-carrying end freely 

 about. If this were the original habit of the primitive 

 tape-worm it would be certain that the so-called head was 

 really the posterior end. 



In any case it is certain that the proglottids become 

 detached, and these may be regarded as asexually produced 

 individuals, which are themselves sexual, so that we have 

 here an example of metagenesis as in Obelia. 



11. Other Tape- Worms. Another tape- worm which 

 frequently occurs in man is Taeniarhynchus saginata, 

 sometimes called Taenia mediocanellata, which differs from 

 Taenia solium in the absence of the circlet of hooks and 

 rostellum ; it has the four suckers only. It is larger than 

 Taenia solium, reaching a length of four metres, having 

 1,200 proglottids, the 600th being the first fully developed. 

 This is the beef tape- worm, its cysticercus inhabiting the 

 flesh of the ox, and infection being due to eating infected 

 beef insufficiently cooked. 



Man may also serve as first host in the life-history of 

 tape-worms, the larvae developing in his body instead 

 of the adult. The most dangerous case of this kind is that 

 of Echinococcus, a bladder-worm, which develops new 

 bladders by successive internal gemmations, the bladders 

 developing very large numbers of heads or scolices. The 

 proliferating Echinococcus usually develops in the liver, 

 and may grow to such an extent as to cause extensive 

 destruction of neighbouring organs and cause the death of 

 the host. The adult form of this larva is Echinococcifer, 

 a very small tape-worm with four suckers and a circlet of 

 hooks, but having only four proglottids and a total length 

 of 5 mm. It lives in very large numbers in the intestine 

 of the dog, wolf, and jackal. The infection in man occurs 

 principally in Iceland and Australia. 



