38 



THE INVERTEBRATA 



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

 fauna: they include many flagellates, Umax amoebae (p. 63), 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 else- 

 where. It has important effects upon the fertility of the soil, by de- 

 vouring valuable bacteria. Perhaps the only truly subaerial members 

 of the phylum are certain mycetozoa. 



Pai-asitic 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. 107), ecto- 

 parasitic (as Oodinium, p. 49), inhabitants 

 of internal cavities (as Opalina, p. 99), 

 tissue parasites (as Myxobolus, p. 95), 

 or intracellular (as Plasmodium, p. 86). 

 They show, according to their degree of 

 parasitism, the same peculiarities as other 

 parasites — reduction of organs of loco- 

 motion, 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 sub- 

 stances, as the malaria parasites do. Many 

 are specific to a particular host or hosts : 

 not infrequently there are two successive 

 hosts belonging to different phyla; thus 

 Aggregata passes from the crab to the 

 octopus, the malaria parasite from man 

 to the mosquito. 



Symbiosis of various kinds is practised by both holophytic and holo- 

 zoic protozoa. Instances of this are described below, on pp. 42, 63, 

 104. 



The division of the phylum into the four classes, Sarcodina, Mastigo- 

 phora, Ciliophora, and Sporozoa, characterized by the presence or 

 absence in the predominant phase of the life history of the several 

 types of motile organs, will be familiar to the student. Two attempts 

 have been made to brigade these classes into subphyla. One con- 



Fig. 34. Oodinium poucheti, 

 parasitic on Oikopleura. A, An 

 Oikopleura bearing the para- 

 sites. 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". 



