PROTOZOA 



43 



conditions at home are no doubt due to the faciUty with which they 

 form resistance cysts (p. 22). In various cases all the unsuitable con- 

 ditions of the environment indicated above have been found to induce 

 encystment, and encysted protozoa have been discovered in dust from 

 the most remote desert regions. 



The protozoa which live in dung (coprozoic species) and in decaying 

 bodies, and those of very foul waters, are branches of the aquatic 

 fauna: they include many flagellates, Umax amoebae (p. 69), and 

 ciliates, and the conditions in which they are in the active state may 

 exist only for a very short period. These faunas merge on the one hand 

 into that of intestinal parasites, and on the 

 other into that of damp earth. In the latter 

 there is a large population, some of whose 

 members {Euglena, Arcella, Paramecium^ 

 etc.) are of common occurrence elsewhere. 

 It has important effects upon the fertility 

 of the soil, by devouring valuable bacteria. 

 Perhaps the only truly subaerial members 

 of the phylum are certain mycetozoa. 



Parasitic members are included in nearly 

 all the principal divisions of the phylum, 

 but not in the Radiolaria or Volvocina. 

 The Sporozoa are exclusively parasitic. 

 The relations of parasitic protozoa to their 

 hosts are of all degrees of intimacy : they 

 may be merely epizoic (as Spirochona, 

 p. 114), ectoparasitic (as Oodinium, P- 55)j 

 inhabitants of internal cavities (as Opa- 

 lina,p. 1 06), tissue parasites (as Myxobolus, 

 p. 100), or intracellular (as Plasmodium, 

 p. 91). They show, according to their de- 

 gree of parasitism, the same peculiarities 

 as other parasites — reduction of organs of 

 locomotion, simplicity of form, means of 

 fixation, the liberation of numerous young (in the Sporozoa), etc. 

 Some, as Entamoeba histolytica, are harmful by destroying for their 

 own nutriment the tissues of the host : more by secreting poisonous 

 substances, as the malaria parasites do. Many are specific to a par- 

 ticular host or hosts. Not infrequently there are two successive hosts 

 belonging to difl^erent phyla: both of these may be invertebrates, as 

 with Aggregata, which passes from the crab to the octopus, but more 

 often one is a vertebrate and the other an invertebrate. In such cases 

 it is often possible to decide which was the original host, and this 

 proves sometimes to be the vertebrate and sometimes the inverte- 



Fig. 34. Oodinium poucheti, 

 parasitic on Oikopleura. A, 

 An Oikopleura bearing the 

 parasites. B, A free spore of 

 the parasite, pst. parasite, on 

 the tail of the host. The trunk 

 of the Oikopleura is enclosed 

 in the newly-secreted and not 

 yet expanded "house". 



