40 THE INVERTEBRATA 



circumstances. The reaction is then repeated. Thus the organism is 

 shepherded by its reactions in the direction of the optimum con- 

 ditions. Fig. 30 shows the path of an individual in the neighbourhood 

 of a particle whence is diffusing some substance of which a certain 

 concentration is optimal for the species to which the individual 

 belongs. Any departure from this concentration turns the moving 

 individual, so that it is led to and kept in a zone in which the optimum 

 exists. Fig. 31 shows how members of a culture of the flagellate Bodo 

 sulcatus, when placed under a coverslip, find by this reaction the 

 optimum concentration of oxygen, which is at first in the middle of 

 the field and recedes as the organisms use up the supply of the element. 



The number of ways in which a protozoon can respond to stimuli 

 is at most small, but the response to a stimulus by an individual in 

 many cases depends not only upon the nature of the stimulus but 

 also upon the condition of the individual at the moment (hunger, 

 fatigue, etc.). 



The relation of protozoa to their environment is governed primarily 

 by the fact that, owing to their small size, any cuticle which is thick 

 enough to protect their protoplasm from loss of water or poisoning 

 by substances in the medium has the effect of immobilizing the 

 organism. Hence in the active phase they are only found in water or 

 in damp places on land, and are peculiarly susceptible to variations in 

 the composition of the medium. Purely holophytic protozoa are also 

 dependent upon the presence of sunlight. Save for these restrictions, 

 members of the phylum are found in every environment in which any 

 other species of organism can exist. In the sea they are plentiful alike 

 in the plankton and in the benthos, and occur at all depths. Their 

 planktonic members are liable to possess the same peculiarities which 

 appear in members of other phyla in the same conditions — spininess 

 (Figs. 6C, 67, 69), phosphorescence, buoyancy, etc. In attaining a 

 low specific gravity they often show an expedient of their own, namely 

 the presence in their protoplasm of vacuoles of water of lower saline 

 content than the medium in which they are suspended (radiolarians, 

 Glohigerina, heliozoa ; Figs. 32, 33, 69, 71). In fresh waters their species 

 have the same cosmopolitan distribution as other small freshwater 

 organisms. Most of them, however, are severely restricted, in all 

 the localities in which they are found, by the necessity for conditions 

 which only occur in some one type of environment, and often even 

 there only during certain seasons or (as in the case of the dung fauna) 

 for yet siiorter periods. In this matter protozoa are particularly subject 

 to the/)H of the medium, its dissolved organic contents, and its saline 

 contents. Thus Po/_y/om« flourishes in an acid medium, ^S^zVoj-Zowwrn re- 

 quires a slightly alkaline one, and Acanthocystis pronounced alkalinity. 

 Euglena viridis and Polytoma live in highly nitrogenous mi\is\ons,Acti- 



