174 REPRESENTATIVE SINGLE-CELLED ANIMALS 



conjugating cells of protozoa are collectively termed gametes, as are 

 also the sex cells, ova and spermatozoa, of metazoa. Likewise, 

 conjugation and fertilization, or union of ovum and spermatozoon, 

 are collectively termed syngamy, meaning union of gametes. The 

 single cell formed by syngamy is termed the zygote, which means 

 something " j^oked." In the case of euglena above mentioned, the 

 gametes are all ahke and hence are called isogametes. In some 

 protozoa (Fig. 108, p. 208) they are dissimilar, Uke the spermato- 

 zoon and ovum of metazoa, and are therefore called anisogametes. 

 Syngamj' may thus be a process of isogamy, by the union of 

 isogametes, or a process oianisogamy,hyihe union of anisogametes. 

 Such smaller and larger conjugating cells in protozoa are called 

 viicrogametes and macrogametes, respectively, and are comparable 

 with spermatozoa and ova (c/. Fig. 212, p. 402). The phe- 

 nomenon of sex, which consists in the production of two kinds 

 of gametes, is recognizable in protozoa wherever there are micro- 

 gametes that are smaller and more active, like spermatozoa, 

 and macrogametes that are larger and tend to be food-laden, 

 like the ova of metazoa. Thus in the euglena and other protozoa, 

 reprodudion, which max be defined as the formation of a new 

 indi\'idual, may be asexual, by means of cell di\'ision, or sexual, 

 b}' means of conjugation. 



Other Mastigophora. — The flagellated protozoa are difficult to 

 subdi\'ide into lesser groups because of the diversified habits and 

 structures of various species. On the one hand, there are flagel- 

 lates, the Phytomastigina, so plant-like that many of them are 

 classified as plants; on the other, there are forms, the Zoomasti- 

 gina, that are distincth' animal-like in their nature. In addition 

 there are forms, like the genus Mastigamxxha (Fig. 85), in which 

 the cell is amoeboid and resembles the Sarcodina, although the 

 presence of a flagellum leads to the classification of such genera 

 with the ]Mastigophora. There are also many instances of amoe- 

 boid stages in the life cj'cles of species that are typical flagellates 

 throughout the greater portion of the life histon,'-, a condition 

 which may be compared with the occurrence of flagellated stages 

 in the hfe cycles of certain Sarcodina. These facts suggest an 

 evolutionary relationship between Mastigophora and green plants 

 on the one hand and Mastigophora and Sarcodina on the other. 



Turning to specific examples of Mastigophora that are of inter- 

 est, there are many simple flagellates occurring in fresh water 



