78 LIFE IN THE SEA [PART I 



chromatophore, the structure containing the chlorophyll essential 

 for the metabolism of the organism. This cell is enclosed in a 

 siliceous shell consisting of two valves which fit over each other 

 in much the same manner as the lid of a pill-box fits on to the 

 box. Reproduction is carried out by the division of the cell 

 within the shell. After the cell divides two new valves are 

 formed, one valve within each of the two which formed the 

 original shell of the diatom. It follows from this that at each 

 division the daughter diatoms must be smaller than the mother 

 cell, and further that this process cannot continue indefinitely. 

 This is the case as a matter of fact. After a time the process of 

 reproduction by division is superseded by a process, in the course 

 of which two diatoms of the same species come together and 

 become surrounded by a gelatinous investment. The contents of 

 the two shells, that is the diatom cells, then issue from the shells 

 and mingle together and a structure known as an auxospore is 

 formed and this becomes a diatom of the typical form in which 

 the process of division is again set up. When circumstances 

 become unsuitable, as when the w^ater in which the diatom is 

 contained dries up, the cell can form a resting spore, that is the 

 protoplasmic contents become gathered up into a compact mass 

 which is able to resist unfavourable conditions. 



An almost infinite variety of shape is exhibited by the shells of 

 diatoms. We find that those resident in fresh water have forms 

 which are quite typical of this habitat and differ notably from 

 those of the marine species. Navicula, for instance, is a type of the 

 bottom living diatoms both of the sea- and of fresh-water. In both 

 the fresh- water and sea-water flora we have the peculiar stalked 

 forms Gomphonema and others. In the mud are the littoral 

 genera Navicula and Pleurosigma with an incredible number of 

 species. Both of these forms are diatoms shaped somewhat in the 

 fashion of little double-stemmed boats. They can move about in 

 a graceful manner and by means of a mechanism the precise 

 nature of which the botanists do not yet seem able to determine, 

 in spite of the enormous amount of work which has been done on 

 these organisms both by amateur and professional workers. In 

 the sea the principal pelagic forms are Coscinodiscus, a diatom 

 Avhich is shaped like a pill-box, and some species of which are 

 relatively large ; Biddulphia, a diatom the typical form of which 



