PHYLUM PLATYHELMINTHES 



283 



relationship is traceable. Oaryophyllceus (Fig. 209) is a Cestode 

 which, but for the absence of an enteric cavity and the want 

 of organs of adhesion at the posterior end, is not far distant 

 from the Trematodes ; and the same might be said of Gyrocotyle 

 (Fig. 210), Amphilina, and Archigetes (Fig. 21 1). 1 The most 

 important difference between a Cestode and a Trematode, in 

 addition to the absence of an enteric cavity in the former and 

 its presence in the latter, is the occurrence in the Cestodes of 

 strobilation. Ligula in a certain sense forms a connecting link in 

 this respect between the Trematode and the ordinary Cestode, the 

 body being elongated and the reproductive organs repeated as in 



Monogenetica 



Nemertinea 



Polycladida 



\Temnoc phalea 

 Tricladida 



Rhabdocoelida 



Digenetica Polyzoa 

 Monozoa 



Lower Coelenterata 

 Fio. 232. Diagram of the relationships of the Platyhelmlnthes (together with the Nemertinea). 



the normal Tape- Worm, but there being no corresponding division 

 of the body into a string of definitely separated proglottides. 



Of importance in connection with the subject of the relationship 

 of Trematodes and Cestodes is the question whether the scolex of 

 the latter is at the end corresponding to the anterior end of the 

 former, or whether it is the free end of the strobilia that is in 

 reality anterior. In favour of the latter conclusion is the fact 

 that the hooks of the hexacanth larva, developed at its anterior 

 end, are found in the cysticercoid to lie in the tail region, i.e., the 

 region most remote from that which develops the scolex, and thus 

 at the end which should represent the free extremity of the 

 strobila. On the other hand, the specialisation of the nervous 

 system to form quite definite and comparatively elaborate nerve- 



1 It is possible, however, that in the last two forms we have, to do with 

 larval Cestodes which have failed to reach the mature stage, and have under- 

 gone a precocious development of the sexual apparatus. 



