POLYMORPHISM 23 



the ciliated area in which the cytostome lies may be regarded as the ventral 

 surface, and the filament of attachment as a development from the apex 

 of the cone-shaped dorsal surface. 



POLYMORPHISM.— It has to be recognized that amongst the Protozoa 

 variations in the shape and form of the body occur at different 

 stages of development. Such a variation is not a characteristic feature 

 of the Rhizopoda, for the smallest individuals of any species have essen- 

 tially the same body form as the fully-grown larger ones. Amongst the 

 Dimastigamoebidse at certain stages of development one or more flagella are 

 formed. Though it is purely an arbitrary matter whether the flagellate 

 stage is considered to be the adult form or not, these amoebae are definitely 

 polymorphic (Figs. 119 and 120). As all Rhizopoda, however, are able 

 to encyst under certain conditions, the encysted stage has to be recognized 

 as another form in which any particular amoeba may occur. Amongst 

 the free-swimming Ciliata, again, the smallest individuals differ little 

 except in size from the fully -grown largest forms. 



Protozoa which only show a slight degree of variation in body form 

 during their life-cycle are termed monomorphic, to distinguish them from 

 polymorphic forms, which show this to a marked degree. This poly- 

 morphism is well illustrated by the development of the Suctoria (Fig. 532). 

 Amongst these Protozoa th e attached adult buds off a small ciliated embryo, 

 which, after leading a free-swimming existence for a time, finally attaches 

 itself, loses its cilia, and grows into the adult, which is provided with 

 sucking tentacles. As the ciliated stage is only of a temporary nature, 

 and is small when compared with the tentacled stage, it is regarded as the 

 embryo. Amongst the Sporozoa there is a high degree of polymorphism 

 associated with their complicated cycles of development. In the case of 

 the malarial parasites, for instance, the organism passes through a constant 

 series of changes of form (Fig. 391). The minute amoeboid organism 

 within the red cell grows into the schizont, which breaks up into elongate 

 merozoites, which again become amoeboid forms in other cells. Some 

 merozoites develop into gametocytes of two types, which can be distin- 

 guished from the schizonts. In the mosquito the gametocytes change 

 in character and produce elongate vermicular zygotes, which pass through 

 the stomach wall and develop into oocysts, which again produce a large 

 number of minute sickle-shaped sporozoites, which differ in character from 

 the merozoites. In this case, as in other Sporozoa, there is a high degree 

 of polymorphism, as exhibited by a constant series of changes in the size 

 and form of the body. Very frequently there is a polymorphism associated 

 with the occurrence of a sexual process and the formation of gametes. 

 In the gregarines the gametes which unite may be exactly alike, in which 

 case the process is known as one of isogamy. On the other hand, in certain 



