UNSEGMENTED ‘‘WORMS.” 1s 
(1) Cheetopoda or Bristle-footed worms, ¢.g. earthworm. 
and lob-worm ; and (2) Hirudinea or Leeches. 
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Tf, Wa 
Ge? 
Unsegmented ‘“worms.”—These differ from the higher 
“worms” in the absence of true segments and appendages, 
and resemble them in their bilateral symmetry. There is 
a motley lot :—the free-living Turbellarians or Planarians ; 
the parasitic Trematodes or Flukes ; the parasitic Cestodes. 
or Tape-worms; the Nemer- 
teans or Ribbon-worms; the 
frequently parasitic Nematodes 
or Thread-worms; and several 
smaller classes. 
As to some other groups, 
such as the sea-mats (Polyzoa 
or Bryozoa), the lamp-shells 
(Brachiopoda), the worm-like 
Sipunculids, and the wheel- 
animalcules or Rotifers, we 
must confess that they are still a, Early stage with head inverted. 
incert@ seats. &, Later stage with head everted. 
But the general fact is not 
without interest, that in the midst of the well-defined 
classes of Invertebrates there lies, as it were, a pool from 
which many streams of life have flowed; for among the 
heterogeneous “worms” we may find in diverse types. 
affinities with Arthropods, Molluscs, Echinoderms, and. 
even Vertebrates. 
Contrast of Coelomate and Ccelenterate.—At this stage 
we may notice that in all the above forms the typical symmetry is. 
bilateral (in Echinoderms, the superficial radial symmetry belongs 
only to the adults); that in most types a body cavity or coelom 1s. 
developed ; that the embryo consists of three germinal layers (external: 
Fic. 15.—Bladderworm stage 
ofa Cestode.—After Leuckart. 
