ANIMAL-PLANTS AND SEA-WEEDS 115 



Norfolk, moderately magnified. These zoophytes, 

 or "animal-plants," abounded in the primitive 

 seas, fulfilling the same functions as the many 

 living zoophytes of to-day : separating the car- 

 bonate of lime from the waters, to build habita- 

 tions for themselves ; and at the same time, by 

 their myriads of accumulating skeletons during 

 countless ages, unconsciously raising materials 

 which afterwards would be quarried as solid rock 

 for the dwelling-places of man himself. 



As is well known, our limestones were built up 

 chiefly by various tiny living organisms which 

 possessed this power of separating the lime from 

 the waters. Fig. 75 illustrates the structure of a 

 minute portion of a very thin slice of limestone, 

 showing how it is built up of what were once living 

 animals. In this particular specimen zoophytes 

 are not prominent ; but each limestone varies 

 according to the locality and period in which it 

 had its origin. 



It is easier to understand how masses of rock 

 substance may be built up by the united efforts of 

 minute zoophytes, after examining one of those 

 Mediterranean corals that now constitute quite an 

 article of commerce ; the harvesting of which 

 affords employment to some thousands of seamen 

 and a large number of vessels especially fitted out 

 for the gathering of these beautiful works of 



