CHAPTER VI 
PLATYHELMINTHES 
{ Acoela—Convoluta, Proporus. 
Rhabdoceclida Rhabdocoela—Mesostoma, Mi- 
crostoma, Vortex. 
TNE \ Alloiocoela—Plagiostoma. 
PLATY HEL- Tricladida—Planaria, Gunda. 
MINTHES Dendrocoelida ~ Polycladida—Thysanozoon, 
Yungia. 
CrstopA—Taenia, Bothriocephalus. 
Monogenea—Aspidogaster, Polystomum. 
LE DA : . ; ; } 
TREMATO Digenea—Distoma, Fasciola, Bilharzia. 
® 
The three classes included under this heading—the TURBEL- 
LARIA, the TREMATODA, and the CestopA—incelude those animals 
which stand in the nearest relationship to the Coelenterata. 
CHARACTERISTICS.—They are not very typical members of the 
sub-grade Coelomata, because, although they possess a distinct 
mesoderm, the coelom itself is reduced to mere splits and 
regular cavities between the mesodermic cells. The absence 
of any well-developed coelom may, however, be due to de- 
generation. The PLATYHELMINTHES ave further characterised 
by the absence of a distinct vascular system, and the alr- 
mentary canal has no anus. The nervous system consists 
of a plexus 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 commuissures. 
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. 
