i82 THE BIOLOGY OF THE SEA-SHORE 



the sea, since it composes the skeletons of large numbers 

 of both marine plants and animals such as diatoms, Radio- 

 larians, Sponges, and others. Diatoms, in particular, 

 contain a very large proportion of silica and, in fact, the 

 frequent presence of this largely indigestible substance 

 would seem to have had important consequences, witness the 

 various means which are found for dealing with it. In 

 calling silica largely indigestible, we are not unaware of the 

 part this substance plays in animal metaboHsm. We know, 

 for instance, that it is generally distributed, albeit in minute 

 quantities, throughout human tissues and is also of regular 

 occurrence in the urine. The fact nevertheless remains 

 that silica occurs in marine diatoms in far greater quantities 

 than can ever be utilised by animals. Moreover, it seems 

 almost certain that marine animals obtain their supplies of 

 silica in the same way as do the plants, viz. by absorption 

 direct from solution in the sea-water. So small is the 

 proportion of silicic acid or silicate present in the sea that 

 there is considerable difficulty in conceiving of the manner 

 in which organisms, such as diatoms, are able to extract 

 sufficient for the manufacture of their skeletons, a task 

 which would seem to require the expenditure of an almost 

 impossible amount of energy. This difficulty, Johnstone 

 suggests, has its origin in the natural tendency to regard the 

 metabolic processes of the lower invertebrates, of which 

 we know little, as more or less resembling those of mammals 

 about which we know so much more. An organism such 

 as a diatom or protist absorbs food over the whole of its 

 surface and not through an alimentary canal. Further, the 

 smaller the organism the greater the proportion of surface- 

 area to volume, and consequently the larger the amount of 

 dissolved substances it is able to absorb. 



(5) Calcium occurs in solution chiefly as calcium bicar- 

 bonate and also as calcium sulphate. Lime plays an even 

 more considerable part than silica in the sea, since it is 

 required for bone-formation in fishes and for the formation 

 of shells, tubes, and skeletons in a very great many inverte- 

 brates. Unlike silica, however, lime is present in the sea 



