p. E. Raymond — History of Corals ,- etc. 347 



brachiopods, and the anomalous Archaeocyathinse. The 

 sponges are all of the siliceous types, and only the 

 Archaeocyathinae secrete calcium carbonate in any quan- 

 tity; they are also, aside from the sponges, the only ani- 

 mals which are firmly fixed in one spot. Reproduction 

 by budding in colonial fashion was also in its infancy in 

 Cambrian times, showing incidentally that the sexual pre- 

 ceded the asexual method, and this indicates clearly that 

 animals had but recently ^ ' discovered the ocean bottom. ' ' 

 Colonial life really developed first in Ordovician times, 

 when sedentary animals like the Hydrozoa, Bryozoa and 

 Anthozoa first became abundant. 



Throughout geological history the times of great 

 expanse of shallow, warm seas have been the times of 

 great development of animals with thick shells. Once the 

 lime-secreting habit had been acquired, animals persisted 

 in it. The chief factors in its origin, however, would 

 appear to be excess of salts, due to warming of the 

 waters, and decrease in power of elimination, due to 

 sedentary habits. 



Museum of Comparative Zoology, 

 Cambridge, Mass. 



