PARASITIC FLATWORMS 



425 



tissue of the host. In some tapeworms the head carries long suck- 



ing grooves or bothria, and in others round cup-shaped suckers or 



ace tabula. More complicated hold-fast organs are developed on 



the scolex in certain groups of cestodes parasitic in marine hosts. 



A slight constriction behind the head has been given the name of 



"neck"; many cestodes have no neck. The body usually in- 



creases in caliber from the head toward the opposite end. The 



partition lines of proglottids are at first very indistinct, and be- 



come more marked as one goes backward along the chain. The 



form of the proglottids also changes from the scolex toward the 



other end of the worm. Much has been made of these and other 



minor details of external 



appearance in the descrip- , 



tions of cestodes. They 



are not adequate for the va " 



determination of many 



species and moreover are 



not of fundamental signifi- 



cance. Unfortunately very 



few cestodes are transpar- 



ent and it is not easy to 



study the internal struct- 



ure, since the specimen 



must first be subjected to 



a time-consuming technic. 



Methods for the prepara- 



tion of cestode material 



were outlined briefly in the 



general section of this chap- 



ter (page 368). Specimens 



must be kept flat and ex- 



tended or they are difficult fla ^ e G ne ^ sho ^f 3&Sgf* of 



to study and interpret cor- 



p JBftSli 



COT, excretory vessel, ventral; nt, lateral nerve; ooc, oocapt; 

 00 ovary; ut, uterus; Da, vagina; vi, vitellaria. X 52. (Af- 



Each proglottid may be * 

 considered as a unit of structure as it contains a complete set of 

 reproductive organs (Fig. 730). With rare exceptions tapeworms 



