63 



the vernal pulse, of the midsummer maximum of the chlorophyll- 

 bearing Mastigophora, and of the autumnal- winter wave of Ciliata. 

 The Protozoa, through the Mastigophora, share with the algas 

 the synthetic function in the elaboration of food from inorganic or 

 partially disorganized organic contents of the water. _They utilize 

 decaying organic matter as food, and are thus primary links in the 

 cycle of food relations. Some of them feed upon bacteria, upon alga3, 

 or even upon other animals, and thus become secondary or tertiary 

 links in the chain. 



MASTIGOPHORA. 

 (Plates I. and II.) 



Average number, including, without duplication, both silk and 

 filter-paper collections, 95,856,449. In the collections of 1897 

 they were five times as abundant as a result, in part at least, of the 

 extended low-water period, sewage contamination, and extension 

 of high temperatures during the late autumn of that year (Pt. I., 

 PI. XL). 



The Mastigophora abound in every collection and occur at all 

 seasons of the year. Four fifths of them occur, however, between 

 the first of April and the last of September. They are predominant- 

 ly chlorophyll-bearing organisms, and have their greatest numbers 

 during the same season in which the land flora attains its growth. 

 They spring into abundance with the opening buds of April, and van- 

 ish from the plankton when frost cuts off the foliage in autumn. 

 There are, it is true, some species, such as Synura, which grow 

 luxuriantly at winter temperatures, but these are generally of the 

 chrysomonad type, with yellowish or brownish chromoplasts. The 

 bright green chlorophyll-bearing flagellates are in the main summer 

 planktonts. Since water temperatures do not fall below 32, the 

 phytoplankton is exempt from this risk of destruction against which 

 the land flora must provide. We find, accordingly, that the most of the 

 Mastigophora are wont to occur in diminished numbers and irregu- 

 larly in the plankton throughout the winter. This appears in the 

 records of the more common species, and fuller examination would 

 doubtless greatly increase the number which thus winter over in 

 reduced numbers. 



I have already called attention to the fact that there are in 

 1898-99 recurrent pulses in the Chl0rophyce& and Bacillariacece at 



