130 UNSEGMENTED “WORMS.” 
Puy_LuM PLATYHELMINTHES 
The Platyhelminthes or flat-worms tnclude three chief classes 
—Turbellarians, Trematodes, and Cestodes—which form a 
velated series. The body ts flattened from above downwards ; 
the mesoderm forms a compact mass of cells or parenchyma 
without a definite celom; there ts the beginning of a head- 
brain ; the excretory system consists of a pair of lateral canals, 
gluing off many branches, whose twigs end in peculiar “flame- 
cells” ; almost all are hermaphrodite. 
There is no doubt that the three classes, Turbellarians or 
Planarians, Trematodes or Flukes, and Cestodes or Tape- 
worms, are related to one another. A fourth class of 
Temnocephalids must also be admitted. It is interesting 
to notice that the Turbellarians and Temnocephalids are 
free-living, except in the case of a few marine Turbellarians 
which have taken to parasitism; that the Trematodes are 
all parasitic, either external hangers-on (ectoparasites) or 
internal boarders (endoparasites); and that the Cestodes 
are altogether endoparasitic. It is probable that the flukes 
and tape-worms arose from Turbellarian-like ancestors which 
adopted parasitic habits. Attention must be directed to 
the flame-cells which are characteristic of Platyhelminthes. 
Each terminal twig of a branch of an excretory canal 
leads into a large hollow cell, from the base of which 
a bunch of cilia—with rapid movements suggesting a 
flickering flame — projects into the cavity towards the 
lumen of the twig. . 
Class TURBELLARIA, Planarians, etc. 
Turbellarians are unsegmented “worms,” usually leaf-like, 
living in fresh, brackish, or salt water, or in moist earth. 
Almost all are carnivorous, a few are parasitic. They re- 
present the beginning of definite bilateral symmetry. 
The ectoderm is ciliated, often glandular, often with peculiar 
rod-like bodies (rhabdites) which may be discharged on irrita- 
tion, A pair of ganglia in the anterior region give off 
lateral nerve-cords, and there are usually simple sense organs. 
The food canal has a protrusible muscular pharynx, ts often 
branched, and ts always blind. There are no special 
