24S 



PLATHELMINTHES 



number of these varies from smaller forms (Tcenia echinococcus, 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 hinder end of the scolex, by a kind of budding. This explains 

 the well-known fact that 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 the 

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



to considerable size, and separate from the hinder 

 end of the chain and live independently when a 

 certain development is reached. For example, 

 the young proglottids of the human tapeworm, 

 Tccnia soli-urn, 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 parenchyma 

 consists of cortical and medullary substance. 

 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 cerebral ganglion (fig. 223), 

 sometimes fused to a single mass by the great 

 development of the commissure. From the brain 

 two principal nerves run backwards, usually near 

 the edge of the proglottids (fig. 228, N}. The 

 water-vascular system begins with a capillary 

 network richly provided with flame cells. It ex- 

 tends through head and proglottids; usually four 



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

 The two chief trunks are frequently connected by a cross-trunk on the 

 hinder margin of each proglottid (fig. 228). 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. Most 

 important of the adhesive organs are the suckers (acetalntla] ; less import- 

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

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



FIG. 223. Nervous 

 system of Monczia (after 

 Tower). a, sucker.?; e, 

 excretory tubes; g, cere- 

 bral ganglia. Nerves 

 black. 



