242 CLASSIFICATION AND ORGANIZATION OF ANIMALS 



together as the Enterozoa, it means that they are regarded as 

 closely related in their ancestry. It is, therefore, possible to 

 regard a table of classification like the one under discussion from 

 the standpoint of evolutionary development. 



Referring to the accompanying table (Fig. 117) as though it 

 were a family tree and to Fig. 118, one may say that the first great 

 step in evolution was the divergence between the forms of animal 

 life that continued in a unicellular state and gave rise to the Pro- 

 tozoa, and those that acquired the many-celled state and pro- 

 duced the Metazoa. Within the latter hne, the next great diver- 

 gence was between forms that continued in a primitive state and 

 without a gut cavity, the Parazoa, whose principal descendants are 

 the Porifera; and those that acquired such a cavity and became 

 the ancestors of the Enterozoa. Next came the acquisition of 

 a coelome by forms that may be termed Coelomocoela; while 

 the more primitive state survived in the Enterocoela, from 

 which the ccelenterates, and perhaps the platyhelminthes, have 

 descended without fundamental changes in their general organ- 

 ization. Among Coelomocoela, there was then a divergence into 

 the ancestors of the great phyla, and following this a specializa- 

 tion within the limits of the several phyla. Thus ended the most 

 profound changes in evolution, as expressed by the classification 

 indicated. If such changes occurred, they must have taken place 

 at a very remote time, since the earliest fossil-bearing rocks that 

 contain an abundance of life show representatives of all the larger 

 groups except the Chordata {cf. Fig. 259, p. 491). If one desires 

 to speculate upon the problem, some such answer as the one just 

 given will probably be accepted by most biologists, although it is 

 recognized that such conclusions regarding major evolutionary 

 changes are speculative and not to be taken as clearly demon- 

 strable. 



The lesser phases of evolutionary history, for example, the 

 evolution of lesser groups of vertebrates {cf. Fig. 276, p. 515), may 

 be accepted with a higher degree of certainty. The table of cell 

 cycles discussed in the preceding chapter may be consulted in this 

 connection, since it shows how Metazoa might conceivably have 

 arisen from a unicellular ancestry in common with the Protozoa, 

 and since it explains the existence of a unicellular stage in the life 

 cycle of every many-celled animal that reproduces by germ cells. 

 The fact that so many members of the phyla that are grouped as 



