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aquatic ILitt 



Crustacea, etc., are more or less abundant. 

 This condition of the fauna and flora 

 merges imperceptibly into a condition of 

 nearly stable equilibrium, in which green 

 plants and animals, under optimum condi- 

 tions of light and temperature, are so ad- 

 justed that for a considerable period a 

 practically self-supporting microcosm 

 exists — but with the balance of nature 

 established neither Protozoa nor Bacteria 

 can ever again attain their maximum 

 abundance. 



Turning now to the protozoan fauna 

 of the infusion. No one, of course, can 

 predict just what forms or what the 

 sequence of forms will be in any given 

 infusion, since both are the result of so 

 many variables. But if several infusions 

 are studied which have been "seeded" 

 with representative material from a num- 

 ber of sources, the general results can be 

 foretold with reasonable accuracy. 



The variety of species of Protozoa 

 which appear in such infusions is so great 

 that it is necessary, at first at least, to 

 concentrate attention on a few typical 

 forms, and neglect the many transient 

 species which appear apparently at ran- 

 dom. The groups or genera to be con- 

 sidered first are Monads, or simple flag- 

 ellates, Golpidwm and Colpoda, Oxytricha 

 and various closely related hypotrichous 

 forms, Paramecium, Vorticella and 

 Amoeba, because all these organisms are 

 present in practically every infusion. 



These forms ordinarily succeed each 

 other in maximum numbers at the sur- 

 face of infusions in the order in which 

 they are named above. That is, first there 

 is a great growth of monads, rising to a 

 sharp maximum and then declining as 

 Colpidum and Colpoda gradually assume 

 the ascendancy. Then creeping ciliates, 

 like Oxytricha and Stylonychia, devour 

 the Colpidia and flourish. Paramecium, 

 which feeds chiefly on bacteria, comes 

 next — and so the series proceeds. 



Although there is this fairly definite 

 sequence of typical protozoan forms at 

 the surface of infusions, the data at hand 

 does not indicate any definite sequence in 

 the middle or bottom regions. The mid- 

 dle seems to be more or less neutral ter- 

 ritory, which is encroached upon by ani- 

 mals from the top and bottom regions. 

 The bottom protozoan fauna is also, until 

 late in the life of the infusion, relatively 

 meagre ; the main types emphasized 

 above, with the exception of certain 

 Amoebae being essentially surface dwell- 

 ers, and seldom resorting to bottom, ex- 

 cept during or after a period of great 

 development at the top. However, there 

 is no invariable correlation between a fall 

 in numbers at the top and a rise in num- 

 bers of the same organism at the bottom, 

 and it seems clear that, in the majority 

 of cases, when a form declines in one 

 region, most of the animals encyst or die. 

 The latter is certainly true for Parame- 

 cium, because many hundreds of passive 

 and dying individuals, affording a feast 

 for Coleps, sometimes may be seen at the 

 bottom among the debris. Again, myriads 

 of cysts of hypotrichous forms are fre- 

 quently found at the bottom as the sur- 

 face decline proceeds. Amoebae among 

 the general types mentioned give some 

 evidence of migrating from the surface 

 to the bottom, which is their chief abode. 

 On the whole, the bottom dwellers are 

 largely independent of those at the sur- 

 face, though it is usually possible to find 

 there a few struggling individuals which 

 have survived from an earlier prosperous 

 surface population. 



It is impossible to discuss briefly the 

 factors determining the relative number 

 and sequence of the Protozoa. Chief 

 among these are, however, food condi- 

 tions, chemical changes due to bacteria, 

 the excretions of the Protozoa them- 

 selves, and the relative reproductive pow- 

 ers of the different forms. The compe- 



