Phylogeny of Animals 505 



members of the Spongida must, in the early history of the 

 group, have assumed a marine hfe, so as to leave their remains 

 in the earliest cambrian strata that are of marine origin. In 

 the process the group became an evolutionary side line, and in 

 persisting to the present day skeletal frame works of horn or 

 lime or silica, often of great intricacy, have been built up. But 

 the group failed to evolve higher organisms than those com- 

 posing it. Here and subsequently reference should be made to 

 the "family tree" facing p. 474. 



A second great phylum — that of the Coelenterata or Entero- 

 coela — includes as its simplest representatives fresh-water gen- 

 era like Microhydra and Hydra, the latter being world-wide 

 in distribution. The comparatively simple cells that build 

 up both of these genera, and that result from segmentation 

 of one egg-cell as in Spongida, indicate that the beginnings 

 of the group resembled colonial ciliate infusorian cells, in which 

 a colonial protozoan life was replaced by a metazoan individual 

 life. We can best explain the history of the entire group if 

 we consider that descendants of the simple fresh- water genera 

 — which may have existed abundantly during mid-archsean 

 times — migrated seaward, and by a process of continuous 

 budding developed the often large colonies of individuals that 

 now mainly characterize the group. These deposited, in or 

 round themselves, chitinous or calcareous secretions as in 

 many Hydrozoa and Actinozoa, or single individuals became 

 much enlarged and succulent as in some Actinozoa, or evolved 

 reproductive pelagic buds or individuals of watery texture, 

 as in medusiform and lucernarioid examples. 



But, for reasons that cannot be enlarged on now, we accept 

 it that these, like the Spongida, became a side line of evolu- 

 tion, and in persisting from the cambrian to the present period 

 have slowly built up reefs and island masses, but have failed 

 to evolve higher organisms than themselves. 



A somewhat similar history attaches to the Echinodermata, 

 except that the earliest ancestral members seem to be entirely 

 unknown, and so our existing ones are all derived secondarily 

 from ancient marine organisms, but whether these were in 



