CHAPTER XIII 



UNSEGMENTED WORMS (Plat-worms, Thread-worms. 

 Rotifers, Polyzoa, Etc.) 



232. It seems desirable, for the sake of convenience and 

 in order to prevent a confusing array of details, to embrace 

 under this head a number of groups of animals which do not 

 have very much in common except their place of uncertainty 

 in the animal kingdom. They are not to be considered as 

 forming a phylum. There is abundant evidence indeed to 

 enable one to believe that four or five distinct phyla are here 

 included. Some of these groups, however, have members which 

 bear more or less striking resemblances to animals belonging 

 to the recognized phyla, especially to embryonic stages of them. 

 These facts render them of great interest to the zoologist, be- 

 cause they furnish grounds for the hope that, through the study 

 of this heterogeneous assemblage, the origin and kinships of 

 the other phyla may be made clearer. The same facts make 

 them unfit objects for extended study in elementary classes. 



233. Points of General Resemblance. — In external form 

 these animals differ very greatly. They may yary from a 

 cylindrical or even a globular form to a thin ribbon-shape. 

 They agree for the most part, however, in having a main 

 axis which in the free-swimming forms is usually horizontal 

 in position, the anterior end of which is structurally distin- 

 guishable from the posterior. There is usually a distinct 

 bilateral symmetry (see §120) which takes the place of the 

 radial symmetry found in the Ccelenterates. In some types of 

 the Ccelenterates there are certain suggestions of bilateral 

 symmetry but never to the complete exclusion of the radial. 

 This is the most primitive group of multi-cellular animals whose 

 individuals move with one end continually foremost and one of 



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