CHAPTER XIII. 

 UNSEGMENTED WORMS (FLAT-WORMS, THREAD-WORMS, ROTIFERS, 



POLYZOA, ETC.). 



227. 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 of animals, although in the past they have 

 often been included by authors with the Annulata (Chapter 

 XV) under the head of Vermes. There is abundant evidence 

 indeed to enable one to believe that four or five distinct phyla 

 are here included. Each of these groups, however, has mem- 

 bers 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 the greatest pos- 

 sible interest to the zoologist, because they furnish grounds 

 for the hope that, through the study of this heterogeneous 

 assemblage, the origin and kinships of all the other phyla may 

 be made more clear. The same facts make them unfit objects 

 for extended study in elementary classes. 



228. Points of General Resemblance. In external form 

 these animals differ very greatly. They may vary 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 116) which takes the place of the 

 radial symmetry found in the Crelenterates. In some types of 

 the Ccelenterates there are certain suggestions of bilateral 

 symmetry but never to the complete exclusion of the radial. 



183 



