WORMS 



I I 



(i) Chaetopoda or Bristle-footed worms, e.g. earthworm 

 and lob- worm ; and (2) Hirudinea or Leeches. 



Fig. 14. — Earthworm. 



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 

 incertce sedis. 



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 Ccelomate and Coelenterate. — 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 ccelom is 

 developed ; that the embryo consists of three germinal layers (external 



Fig. 15. — Bladderworm stage 

 of a Cestode. — After Leuckart. 



a, Early stage with head inverted. 



b, Later stage with head everted. 



