PHYLUM PLATHELMINTHES 117 



Unsolved Problems. — The question of the orientation of the 

 cestode body has never been decided with certainty. Many 

 zoologists maintain that the scolex is the anterior extremity 

 of the chain because here the chief nervous centers are found. 

 Other investigators contend just as stohdly that since in develop- 

 ment from the larval stage the scolex is at the posterior extremity 

 of the larva this scolex must represent the morphological posterior 

 extremity of the adult chain. Another question upon which 

 there is just as radical division of opinion is that upon the deter- 

 mination of what constitutes an individual. Is the entire chain 

 an individual or a colony of individuals? Continuity of nervous 

 and vascular systems throughout the chain with some modifica- 

 tions at the extremities not found in the individual proglottids 

 presents evidences of unity of the entire chain. On the other 

 hand, since the reproductive organs are about the only structures 

 remaining in the proglottids, complete duplication of these in 

 each proglottid gives support to the argument that each proglot- 

 tid is an individual, groups of which have remained united to 

 form a colony as a result of incomplete separation following 

 asexual reproduction to form the chain. 



Organization. — That part of the strobila which is located 

 between the scolex and the proglottids is frequently not divided 

 into segments but as a more or less sharply defined region is 

 termed the neck. This is the budding zone where new pro- 

 glottids are being formed. Thus in age the scolex is the oldest, 

 then come the proglottids in order from the free extremity toward 

 the neck. Few structures are evident in the neck and in the 

 small proglottids most recently formed, for there is a gradual and 

 progressive development of the organs (organogenesis) repre- 

 senting all stages between the fully formed organs of the terminal 

 proglottids and the merest traces of fundaments in the proglot- 

 tids just behind the neck. The sexual organs of the terminal 

 proglottid are the oldest and consequently mature first. Upon 

 reaching full maturity, the gravid proglottids are frequently 

 severed from the remainder of the strobila. 



In many genera the tapeworm continues to live and to produce 

 more proglottids indefinitely just as long as the scolex and neck 

 region remain attached to the wall of the digestive tube of the 

 host. 



Peculiarities of Chain Formation. — In some forms (Ligula), 

 there are no partitions between the proglottids but the organs 



