THE DAWN OF LIFE 



179 



geographical miles, may be said to be entirely 

 composed of shells of Foraminifera imbedded in a 

 paste of still more minute calcareous bodies, the 

 Coccoliths, which are probably products of marine 

 vegetable life, if not of some animal organism 

 still simpler than the Foraminifera. 



There are, however, some sessile examples of these 

 animals which attain to larger dimensions than the 

 free and locomotive forms. As an example of these 

 we may take the Polytrema, which forms little hard 

 red lumps on West Indian corals. Such a creature, 

 beginning life as a little round spot of protoplasm, 

 almost invisible, and protected with a little dome of 

 carbonate of lime for the extension of its pseudopods 

 as it grows in size, adds chamber to chamber in 

 successive tiers till it assumes an appreciable size, 

 all the chambers communicating with each other, 

 while the outer ones are perforated with pores for 

 extension of the pseudopods. In one form {Carpen- 

 teria) the same end is secured by leaving an open 

 space in the middle of the conical mass like the crater 

 of a small volcano. It is with these larger and 

 sessile formes that we must compare Eozoon, though 

 some of its minute structures rather resemble those 

 of some smaller types. 



