24S 



rLATIIELMIXTHES 



number of these \"aries from smaller forms {Twnia'ccliinocofciis, fig. 232) 



with three or four to hundreds or even several thousands, a fact which 



speaks for the enormous size of some species. The proglottids arise 



from the liinder end of the scolex, by a kind of budding. Tliis explains 



the well-known fact tliat one is not rid of the tapeworm, so long as the head 



remains in the host. It also explains the peculiar shape which is almost 



thread-like in front, increasing posteriorly to a broad band, whence tJie 



common name. At first the proglottids are small; they increase with age 



to considerable size, and separate from the Itindcr 



end of the chain and live independently when a 



certain development is reached. For example, 



the young proglottids of the human tapeworm, 



TiCnia solium, are 0.5 mm. broad and o.oi mm. 



long; the ripe proglottids are 5 mm. broad and 



12 mm. (half an inch) long. 



Head and proglottids have certain common 



characters. Their connective-tissue parenchvma 



consists of cortical and medullary sulistance. 



The first contains most of the muscles, the latter 



the other organs. Nerves and water-vascular 



system extend the whole length of the worm. In 



the head is the paired cereliral ganglion (fig. 223), 



sometimes fused to a single mass by the great 



development of the commissure. From tlie brain 



Fig. 22-,. Nervous ^-^y^ principal nerves run backwards, usuallv near 

 system 01 Monczia (^alter 



Tower). 



suckers; 

 g. cc 



Nerves 



the edge of tlte proglottids (fig. 22S, X). The 



excretor}- tubes; £^^ cere- water-vascular system begins with a capillarv 

 bral ganglia. Nerves -,.,,',, '^ 



black. network richly provided with fiame cells. It ex- 



tends tlirough head and proglotliils; usuallv four 

 main trunks are present, two being less developed and sometimes absent. 

 The two chief trunks are fref[uently connected by a cross-trunk on the 

 hinder margin of each proglottid (hg. 22S). The system opens on the 

 posterior edge of the last proglottid, but accessory mouths may occur 

 on other proglottids. 



Scolex and proglottids differ in that the proglottids contain the sexual 

 organs, while the scolex bears the anchoring apparatus, for the latter has, 

 besides producing proglottids, to fasten the worm in the intestines. jNIost 

 important of the adhesive organs are the suckers {acctabiila) ; less import- 

 ant are the numerous hooks, either arranged in a circle or borne 011 

 protrusible and retractile probosces (figs. 224-226). 



