80 LIFE IN THE SEA [PART I 



is in shape something like a pilloAv provided with variously 

 arranged spines at the angles ; Rhizosolenia, a cylindrical diatom 

 of varying length and dimensions ; Chaetocei^os, in which we have 

 a number of short cylindrical units adherent together with spines 

 of various lengths and manners of insertion ; Thalassiosira, in a 

 number of separate units are strung together on a filament ; 

 Bellerochea, Nitzschia, Bacillaria, Asterionella and many others. 

 Some of these are neritic forms but others are oceanic dwellers. 

 In the diatom ooze, which covers more than ten million square 

 miles of sea bottom in the Antarctic, we have a variety of forms 

 the chief of which are species of Fragillaria, Coscinodiscas, 

 Thalassiothrix, Navicula, Actinocyclus, &c. It will be seen then 

 that many genera of diatoms are cosmopolitan in their distribution, 

 and indeed have the most varied habitats. The species of these 

 are of course different in widely different localities. 



Diatoms have always been a favourite study with amateur 

 microscopists because of the ubiquity of their distribution both 

 in fresh waters and in the sea. No form of microscopical investi- 

 gation can be more attractive than that of the diatoms. These 

 organisms present such an astonishing variety in form, and the 

 elegance of their shape as well as the extreme beauty of the 

 markings on the shells allures the student. But in spite of the 

 incredible amount of investigation that has been lavished on the 

 speciography of this group, much remains to be discovered with 

 regard to the reproduction and life processes. We w^ill see later 

 that the study of the diatoms has been taken up in relation to far- 

 reaching questions of the general economy of life in the sea, and 

 that the stimulus afforded by the necessity for more minute 

 knowledge of the metabolism of these organisms can hardly fail 

 to be productive of a great increase in our knowledge of the group. 



This is, of course, only a very brief sketch of the composition 

 of the plankton, and I have not referred to several very interesting 

 sub-groups. There are hosts of planktonic protozoa like the 

 Flagellates, which are of very great interest, both biologically, and 

 from the point of view of the circulation of food matter in the sea. 

 Then there are the anomalous forms, like the Challengerida, 

 which afford quite special problems in speciography. Finally 

 one must not forget to mention that many algae (like THchodes- 

 iiiium) are planktonic. 



