CHAPTEE VI 



PLATYHELMINTHES 



( Acoela — Convoluta, Proporus. 

 )r" " " " "" 



Rhabdocoelida Rhabdocoela-Jfeoiiioma, Mi- 

 I crostoma, Vortex. 

 TUKBELLAE1A<! '^ AUoiocoela-P^iorfoma. 



PLATYHEL-y I ('i:Tic\a.dida,—Planaria;Gunda. 



MINTHES "^ . ^ Dendrocoelida -j Polycladida — Thysanozoon, 



[_ Yungia. 

 Cbstoda — Taenia, Bofhriocephalus. 



Trematoba \ Monogenea — Aspidogaster, Polystomum. 

 \ Digenea — Distoma, Fasdola, Bilharzia. 



The three classes included under this heading — the Tuebel- 

 LAEIA, the Teematoda, and the Cestoda — include those animals 

 which stand in the nearest relationship to the Coelenterata. 



Chaeacteeistics. — Tliey are not very typical memiers of the 

 sub-grade Coelomafa, because, although they possess a distinct 

 mesoderm, the coelom itself is reduced to mere splits and 

 irregular cavities between the mesodermic cells. The absence 

 of any well-developed coelom may, however, be due to de- 

 generation. The PLATYHELMINTHES are further characterised 

 by the absence of a distinct vascular system, and the ali- 

 mentary canal has no anus. The nervous system consists 

 of a plescus of nerves, mostly on the ventral side of the 

 animal. As we ascend in the group the nerves become 

 more concentrated. A central nerve ganglion, the brain, 

 is found which in the higher forms gives off two longitudinal 

 cords which may be connected by transverse commissures. 

 An excretory system, consisting of fine branches, often intra- 

 cellular, which at their inner end terminate in flame cells, 

 at their outer end open to the exterior, is very characteristic 

 of the group. 



