UNSEGMENTED " WORMS. 1 



1 1 



are unsegmented. The former are often called Annelids, 

 and include two chief classes : 



FIG. 14. Earthworm. 



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

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

 are several smaller classes of much interest. 



Unsegmented <k 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 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 

 at this stage confess that they are incerta 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 " there are 

 types which show 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. 35) (in Echinoderms, the radial symmetry 

 belongs only to the adults) ; that in most types a body cavity or 



FlG. 15. Bladderuonn stage 

 of a Cestode. After Leuckart. 



a, Early stage with head inverted. 

 If, Later stage with head everted. 



