UNSEGMENTED "WORMS: 



ii 



are unsegmented. The former are often called Annelids, 

 and include — 



Fig. 14. — Earthworm. 



Chsetopoda or Bristle-footed worms, e.g. earthworm 



and lob-worm ; and 

 Hirudinea or Leeches ; and some smaller classes. 



Unsegmented " -worms." — These differ from the higher 

 " worms " in the absence of true segments and appendages, 

 and resemble them in their bilateral symmetry. The series 

 includes Turbellarians or 

 Planarians ; the parasitic Tre- 

 matodes or Flukes ; the parasitic 

 Cestodes or Tape- worms ; the 

 Nemerteans or Ribbon-worms ; 

 the frequently, parasitic Nema- 

 todes or Thread-worms ; and 

 several smaller classes. 



As to certain other forms, 

 such as the sea-mats (Polyzoa 

 or Bryozoa), the lamp-shells 

 (Brachiopoda), and the worm- 

 like Sipunculids, it seems best, 

 at this stage, to confess that 

 they are incertcz 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 detect 

 affinities with Arthropods, Molluscs, Echinoderms, and even 

 Vertebrates. 



At this stage we may notice that in all the above forms the typical 

 symmetry is bilateral (see p. 33) (in Echinoderms, the radial symmetry 



Fig. 15. — Bladderworm stage 

 of a Cestode. — After Leuckart. 



a, Early stage with head inverted. 

 £, Later stage with head everted. 



