THE PHYLA OF ANIMALS 179 



vertebrates or backboned animals, and the invertebrates, Vertebrates 

 without any spinal column. The vertebrates consti- brates 

 tute a phylum ; but the invertebrates cannot be thus 

 grouped together, since the various phyla which they 

 include are as distinct from one another as they are 

 from the vertebrates. In several important respects a 

 man is more like an earthworm than the latter is like a 

 sea anemone. 



Since the phyla or grand divisions are so important, Disputed 

 one would suppose that all naturalists would long ago p ya 

 have agreed as to their number and limits, if only as a 

 matter of convenience. This is true in respect to 

 several, but others are still in dispute. The questions 

 involved have to do with the amount of difference neces- 

 sary to establish a phylum. Naturally not all are 

 equally distinct, and at some point it must be difficult 

 to say whether a given group should be a phylum or a 

 class, or whether we should compromise and talk about 

 a "subphylum." It is assumed, however, that a 

 phylum must not be "polyphyletic" ; that is, a collec- 

 tion of unrelated organisms, not derived from any com- 

 mon ancestor possessing the characters of the phylum. 

 For this reason the proposal to include the sponges 

 among the ccelenterates appears highly objectionable, 

 since it is improbable that the two groups have any 

 common ancestor nearer than the Protozoa, or one- 

 celled animals. 



We may recognize the following phyla, which are 

 more fully discussed farther on : 



Phylum Protozoa (page 186) 



Animals consisting of single cells, which may however Protozoa 

 be aggregated together in groups. They are all small, tophyta*" 

 and are closely related to the Protophyta or one-celled 



