PAST HISTORY OF THE INVERTEBRATES 303 



cult task. The number of facts that it is necessary to know 

 is so considerable that even today systematic zoologists are 

 not agreed on important details of grouping animals in a 

 complete system of classification. Besides, the doctrine of 

 evolution takes into account the fact that the phyla of ani- 

 mals have not developed in a direct line, but as branches 

 from a previously existing stem. The whole system of 

 animals might be represented by a stem and a series of 

 branches resembling a tree ; but whereas in a tree we can 

 trace down one branch and out any other, in the diagram 

 suggested some of the branches would not be connected with 

 the stem at all, because the organisms which would stand in 

 the connecting places have disappeared both in living and 

 in fossil form. 



However, we can see very clearly that the Protozoa, being 

 composed of a single cell, are the lowest of all animals ; and 

 that the Porifera, with their structure showing no sign of 

 digestive organs, muscle cells, nerve cells, or sense organs, 

 are the simplest of the many-celled animals (Metazoa). 



After considering all facts of structure and development, 

 we may arrange the phyla in a series going from the sim- 

 pler animals to the more complex (see page 307). But in 

 doing this we must keep in mind that in some instances one 

 group may be simpler than another in regard to certain 

 organs or structures but more complex in other respects. It 

 then becomes necessary to decide which of the differences is 

 the most important. Furthermore, it is not unusual to find 

 that some simple animals are really degenerate examples of 

 a group that is relatively complex. 



Invertebrates which resemble the Vertebrates. All the 

 animals which have been discussed in the preceding chap- 

 ters lack a backbone. The name "invertebrate" is there- 

 fore applied to them in contrast with the highest animals, 

 or 'vertebrates." For a long time a few animals which 



