UNSEGMENTED "WORMS." 



II 



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

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



FlG. 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 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 ccelom is 

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



FlG. 15. Bladderworm stage 

 of a Cestode. After Leuckart. 



, Early stage with head iriverted. 

 6, Later stage with head everted. 



