INVERTEBRATA. 323 



safely be said to represent a lower grade of development 

 than any other, or to stand towards it in anything like the 

 position in which, for example, one class of Vertebrata does 

 to another. Moreover, these five great phyla are far from 

 exhausting the list of Ccelomate animals. Numerous other 

 groups some represented by a wide and varied series of 

 forms, others by very few or even by a solitary species 

 remain to puzzle the morphologist. If we accept the doc- 

 trine of a common descent for all animals, we must admit 

 that only the most fragmentary and uncertain evidence exists 

 as to the way in which the ancestors of the great groups 

 diverged from one another. 



