PHYTOPLANKTON 



been used as objects for testing the resolving power of microscope lenses. 

 For example, Piciirosioiiia am^iilatiiDi can be resolved with a lens that can 

 separate about 45,000 lines per inch, but Ainpliiplciira pcllncida needs a better 

 lens, one that separates 93,000 lines per inch. During the period 1880 to 

 about 1920, there were many microscopists who delighted in using their 

 instruments for the sheer joy of it, not particularly keen to make any new 

 contribution to science but to obtain the ultimate optical perfection. To 

 them the diatoms were favourites, not as a rule the planktonic ones, but the 

 bottom-living species from rock pools, fresh water and bogs. They spent 

 many happy evenings with their paraffin lamp, their expensive instruments, 

 changing lenses etc., and arranging their minute 'collections' in strictly 

 geometrical patterns such as those illustrated in Plate IX. Ready prepared 

 slides of these fancy patterns could also be purchased. So also could slides 

 of named diatoms to help the microscopist to identify his own collections. 

 One such slide prepared in 1869 by Moller of Weden in Holstcin contained 

 about 400 different species beautifully arranged in a square of about 5 milli- 

 metres, and he provided a printed catalogue of their names. Few of us these 

 days have the leisure for such academic pleasures ! 



(c) T]u' Diiiofla(^ellates 



Dinofiagcllates are of two main kinds, 

 the 'naked' forms (Fig. 10 and Frontis.; 

 37-39) and the 'thecate' forms, 40-45, 

 which have a strong exoskeleton built of 

 plates of cellulose. These plates are of con- 

 stant shape for each species and can therefore 

 be used as characters for identifying them. 

 All have two flagella, one free, and the other 

 wrapped round a groove called the girdle 

 (Plate IX). Many naked forms live on the 

 sea floor in shallow water amongst the sand 

 grains and mud, though quite a number are 

 planktonic. There are several hundred plank- 

 tonic tliecate species, one of the commonest 

 being Ccrathun tr\pos[Vx:onns. ; 43 , and Pi. IX) . 



Like the diatoms they mostly contain chloroph\ll but it is masked by 

 other pigments giving them a brownish colour instead of green. They can 

 thus utilize the dissolved salts and gases with the aid of sunlight, and in turn 

 form the food of the animals. Many live only this way, i.e. they are auto- 

 trophic, but some can feed, like the protozoa, on minute organic particles in 



37 



Fig. 10. 



Gymnodiniitm vciicficiuiii, 



a dinoflagellatc that produces 



a poison. 



