GENERAL PRINCIPLES 



15 



Foraminifers having carbonate of lime shells and the Radiolarians 

 shells of silica. Though very small, these shells have frequently 

 built up limestone (chalk), or chert beds. 



II. Porifers or Sponges, which are the simplest of the many- 

 celled animals, are sac-like forms supplied with numerous pores 

 or canals through which water containing food circulates to feed 

 the cells. Distinct organs are lacking. Most Sponges have either 

 siliceous or calcareous skeletons. 



III. Coslenterates are also very simple many-celled animals, 

 but they possess distinct mouth, body (or stomach) cavity, and 

 usually have radiating 

 tentacles surrounding 

 the mouth. The 

 canal system of the 

 Sponges is absent. 

 Hydrozoans are little 

 creatures consisting of 

 tube-like sacs with 

 mouth at one end 

 surrounded by ten- 

 tacles. Anthozoans are 

 very much the same, 

 but have a more or 

 less distinct esopha- 

 gus, and have the 

 body cavity divided 

 by radiating vertical 

 partitions. Some Hy- Fl &- 4 



drozoans and Antho- Mod 1 ern Hydrozoans. Part of a colony much 

 , • , enlarged. (From Schuchert's Historical 



zoans colonize and Geology," permission of John Wiley and 

 some do not. Among Sons.) 



the former, the Graptolites (now extinct) are numerous and im- 

 portant in early Paleozoic rocks, while the latter or Corals have 

 always been prominent since pretty early geologic time. 



IV. Echinoderms possess a distinct body cavity which con- 

 tains the digestive or alimentary canal, distinct nervous system, 

 and a water circulatory system. Most Echinoderms are radi- 

 ally segmented and protected by shells. 1 . Pelmatozoans are char- 

 acterized by having segmented stems by which they are attached 

 to the sea-floor or some object during at least part of their existence. 



