56 PRIMITIVE ANIMALS 



stock. There is a small group of marine worms 

 which at one time were thought to represent the 

 primitive ancestral form of the Annelids, chiefly 

 owing to their much greater simplicity of structure. 

 This group includes the little worm Polygordius 

 which does not possess a trace of limbs. It is now 

 very generally recognized that Polygordius is de- 

 scended from a marine Annelid which possessed limbs, 

 and that it is one of a series of degenerate forms 

 which have gradually lost one after another of the 

 distinctive features which characterized their more 

 highly developed Annelid ancestors. The final term 

 in this series of degeneration is represented probably 

 by the Rotifers or Wheel-animalcules, fresh-water 

 organisms which afford a favourite object for micro- 

 scopic study. These animals have lost all trace of 

 limbs, of segmentation, and of a coelomic body-cavity, 

 but we can trace a series from such a form as Poly- 

 gordius through a number of still further simplified 

 types (Dinophilns, Histriobdetta, etc.), until we reach 

 the Rotifers. 



The Appendiculate phylum, from its vast extent 

 and from its including so many hard-bodied animals 

 likely to be preserved as fossils, illustrates, perhaps 

 better than any other Invertebrate phylum, what 

 degree of success may be expected from an application 

 of the evolutionary theory, and to what limitations 

 this procedure is liable. We have seen the important 



