44 THE INVERTEBRATA 



brate. It is interesting that the two most dangerous protozoan para- 

 sites of Man, the sleeping-sickness and malaria parasites, differ in 

 this way (pp. 63, 91). 



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

 zoic protozoa. Instances of this are described below, on pp. 47, 68, 

 m, 193. 



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- 

 trasts the Sarcodina under the name of Gymnomyxa with the other 

 classes, or Corticata^ on the ground that the latter possess a firm ecto- 

 plasm. The other contrasts the Ciliophora with the rest of the classes 

 (Plasmodroma), which lack cilia and a meganucleus. Neither of these 

 systems is satisfactory, for each is probably grounded, not upon a 

 fundamental cleavage of the phylum, but upon the specialization of 

 one branch of it. 



The ancestral group of the Protozoa is probably the Mastigophora. 

 This is fairly evident as concerns the Sporozoa — a class highly 

 adapted to parasitism, and often possessing a flagellated phase — and 

 the Ciliophora, also a greatly specialized group, which possesses in 

 the cilia organs easy to derive from flagella. The Sarcodina, on the 

 other hand, were formerly held to be ancestral to all protozoa, on 

 account of the supposedly primitive condition of their protoplasm. 

 But neither the structure nor the behaviour of amoeboid organisms is 

 really simple; their holozoic nutrition is a less easy process and is 

 much less likely to be primitive than photosynthesis, which is common 

 in the Mastigophora ; the sporadic occurrence of amoeboid forms in 

 various groups of the Mastigophora probably indicates that the latter 

 have more than once given rise to organisms resembling the Sarcodina; 

 and, finally, the Sarcodina very commonly have flagellate young, 

 but the Mastigophora do not have amoeboid young. The Mastigo- 

 phora, indeed, are probably not only the basal group of the Protozoa 

 but also' not far removed from the ancestors of all organisms, for they 

 alone present (and often can alternate) the modes of nutrition both 

 of plants and of animals ; and their characteristic organ, the flagellum, 

 occurs in the zoospores of plants, in bacteria, and in the spermatozoa 

 of metazoa. 



^ The term symbiosis has been used in various senses. It is here applied 

 to all cases of partnership between two organisms of which one lives within 

 the body of the other and both derive benefit from the association. It is 

 sometimes restricted to cases, such as those described on p. 47, in which the 

 infesting partner is photosynthetic. 



