12 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The cells of which the plant is made up, however, do 

 exercise a selective acl ion in retaining; what the plant 

 requires and in excreting what is not wanted. The inter- 

 action of these substances results in the formation of an 

 inert substance which does not interfere with the genera] 

 functions of life. Many other plants form crystals of 

 calcium oxalate, calcium carbonate and other substances 

 in their tissues, and thus get rid of materials which, if 

 retained, would be harmful. 



Now to return to our original question. The animals 

 and plants inhabiting the early seas found themselves 

 adapted to surroundings which favoured their growth. If 

 the surroundings change they must either adapt them- 

 selves to the altered circumstances or contrive by some 

 means to remove those substances which would be 

 injurious, thus retaining conditions which suit them. 



In this way we may assume that lime and other 

 substances were excreted or precipitated. 



Murray and Irvine* lead us a step further. They 

 show that not only the carbonate of lime which enters the 

 sea may be precipitated, but other salts of lime, such as 

 the chloride, nitrate, sulphate and phosphate, may be 

 changed and thrown down as carbonate. This is brought 

 about by the action of the carbonate of ammonia formed 

 from the urea secreted by the organism. 



In this way minute crystals of carbonate of lime may 

 be produced which may be rained down to the bottom of 

 the sea, or they may be retained in the tissues of the 

 organisms either as separate spicules oi- a number of 

 spicules fused together to form a continuous shell or 

 covering. 



We are still wanting information as to the deter- 

 mining causes which produce the carbonate of lime in its 

 *Proo. Roy. Soo. Edra., Vol. XVI 1.. 1889-00. 



