PROTOZOA 45 



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 

 organism we can as yet envisage was multinuclear and photosynthetic, 

 and gave rise independently to the Metazoa and, by reduction of the 

 body, to flagellates, and so to the Protozoa, which on this view are 

 not truly members of the Animal Kingdom. 



Class MASTIGOPHORA (FLAGELLATA) 



Protozoa which in the principal phase possess one or more flagella; 

 may be amoeboid, but are usually pelliculate or cuticulate; are often 

 parasitic but rarely intracellular; have no meganucleus; and do not 

 form very large numbers of spores after syngamy. 



The reproduction of the Mastigophora is in most cases by equal 

 longitudinal fission. The way in which in many of the solitary 

 Volvocina this becomes transverse has been described above (p. 29). 

 In the Dinof^agellata fission is oblique or transverse. The fission may 

 be simply binary or repeated. The number of fissions often varies in 

 the same species, and is usually greater in the formation of gametes 

 than in asexual reproduction. Binary fission in forms which have not 

 a stout cuticle usually occurs in the free-swimming stage, but may 

 take place in a cyst or jelly case, as, for instance, occasionally in 

 Eiiglena viridis. In forms with a stout cuticle, as in the Volvocina, the 

 protoplasm shrinks from the cuticle, which serves as a cyst. Repeated 

 fission usually occurs in a cyst. The^fate of the flagella at fission has 

 been dealt with on p. 30. The mitoses (see p. 25) in this group range 

 from beautiful eumitoses to the extremest cryptomitoses, the latter 

 generally in parasitic forms. Paramitosis occurs in the Dinoflagellata. 



In many genera syngamy is not known to occur. Among those in 

 which it does, all degrees of difference between gametes are found, 



