PROTOZOA 39 



trasts the Sarcodina under the name of Gymnomyxa with the other 

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

 ectoplasm. 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 speciaHza- 

 tion 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 Sarco- 

 dina ; 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 connection between the Protozoa and the Metazoa in the family 

 tree of the Animal Kingdom is an interesting but a very obscure 

 problem. Concerning it three theories are held. The first, supported 

 by the morphological resemblance of the uninucleate protozoon to a 

 cell in the body of a metazoon, and of Volvox to the blastosphere 

 stage in the development of such a body, holds that the metazoon is 

 a colony of protozoa, each differentiated as a whole for some function 

 in the body which they compose. The second, based on the fact 

 that the protozoon, which performs equally all the processes of life, 

 is thus physiologically equivalent not to one cell but to the whole 

 body of a metazoon, holds that the Metazoa arose from multinucleate 

 protozoa by the nuclei taking in charge each a local, differentiated 

 portion of the cytoplasm. The third, based on the fact that, save for 

 their mode of nutrition, the Metazoa have — in their cellular structure, 

 nuclear division, maturation of gametes, etc. — ^more in common with 

 multicellular plants than with the Protozoa, holds that the earliest 



