334 



PLATYHELMIXTIIES. 



Fio. 



'271. Oysticercoid of 

 a cucumerina, magni- 

 fied 60 times (after R. 

 Leuckart). 



itself to the intestinal wall, and grows by gradual segmentation into 

 a tape-worm. From the Scolex the chain of proglottides proceeds as the 

 result of a growth in length accompanied by segmentation, a process 

 which is to be looked upon as a form of asexual reproduction (bud- 

 ding in the direction of the long axis). Since, however, it is the body 

 of the Scolex which undergoes growth and segmentation, it seems 



most natural to assume the individuality of 

 the entire chain, and to subordinate to this 

 the individuality of the proglottides. The 

 development of the tape-worm is then to be 

 explained as a metamorphosis, characterised 

 by the individualization of certain stages of 

 the development. It is only in those cases 

 in which the young form produces a number 

 of heads that the development can be ex- 

 plained as a case of alternation of genera- 

 tions. 



The development of some tape-worms pre- 

 sents considerable simplifications. In the 

 cysticercus stage the vesicle frequently dimin- 

 ishes to an excessively small appendage, and the Cysticercus becomes 

 a cysticercoid form, in which one portion bearing the embryonic hooks 

 is distinct from a larger part which represents the scolex (figs. 271, 

 272). In other cases the embryo, becomes a Scolex directly without 

 passing through a cystic stage, so 

 that the Scolex stage is merely a 

 late stage of the embryo (Hotltrio- 

 cephalus). The segments produced 

 from the Scolex also show very 

 different degrees of individuality, 



and finally are sometimes not deve- 



~^^ s ^g^' i 



loped at all. In the latter case rr 



(Caryop/tyllceus) the head and body 



cannot be sharply distinguished from 



one another, and represent only one 



single individual comparable to a 



Trematode and characterised by its 



single generative apparatus. Its development is to be looked upon 



as a metamorphosis completing itself in one individual. 



Fam. Tseniadae. The armature of the head consists of four muscular suckers, 

 to which is frequently added a single or double circle of hooks on the rosteHiim. 



FIG. 272. Echinococcns-\\\iti Cygticercoid 

 from the body cavity of the Earth- 

 worm (after E. Metschuikoff). </, 

 Brood-capsules with three Cysticer- 

 coids. li, Cysticercoid with evaginated 

 head. 



