86 PROTOZOA 



tions of a nervous system; sensitive areas, which function as sense organs; 

 and supporting parts, which form a sort of skeleton (Fig. 31). 



The food of protozoans consists of organic matter, both vegetable 

 and animal, living and dead. Their metabolism is, in general, similar 

 to that described for the ameba or the Paramecium. Because of the 

 size of the animals the study of digestive enzymes is difficult, and there 

 is little precise knowledge. Protozoans certainly digest proteins, have 

 been shown to be able to use emulsified fats, and also are able to use 

 certain starches. 



In addition to fission, or binary division, and sporulation, protozoans 

 sometimes exhibit a third type of asexual reproduction known as gemma- 

 tion, or budding. In this case individuals of smaller size than the parent 

 grow out from it like buds and when developed break loose, later growing 

 to the same size as the individual which produced them (Fig. 32). 



A B 



Fig. 32. — Gemmation, or budding, in Ephelota gemmipara (Hertwig). (After Hertwig, 

 in Morphologisches Jahrhuch, vol. 1.) A, organism on stalk, showing two types of ten- 

 tacles, suctorial and prehensile, the latter with spiral ridges on the surfaces. B, an indi- 

 vidual showing the formation of buds, into each of which extends a portion of the nucleus. 

 These buds become detached and free-swimming; they possess cilia on one side but later 

 develop tentacles and become attached. X 120. 



112. Sexual Reproduction in Protozoa. — Some colonial protozoans 

 exhibit a simple form of sexual reproduction. The animals in the colony 

 become divided into two types: the ordinary ones, known as nutritive 

 individuals, or nutritive zooids, which reproduce by fission in the ordinary 

 way; and a second type which is represented by reproductive individuals 

 or gametes. These gametes exist in two forms : the larger macrogametes, 

 which, like egg cells, are usually not active; and the smaller microgam- 

 etes, which, like the sperm cells of higher animals, are active. When 

 these two types of sex cells unite, a zygote is formed from which a new 

 colony may arise. The macrogametes may also show a type of sexual 

 reproduction without fertilization. When this occurs, they' remain 

 within the colony, increase in size, divide into many cells, and finally 

 escape to form new colonies. These groups of cells are known as parthe- 

 nogonidia (Fig. 27 E). In many of the Sporozoa there are both sexual 

 and asexual generations. The zygotes produce a number of spores which 

 develop into sporozoites. These become nutritive individuals, or tro- 

 phozoites, and these in turn may form another generation of gametes. 



