130 PHYSIOLOGY OF PROTOZOA 



existence recur. Within the cysts the organism either undergoes no 

 change or it may continue to multiply. In parasitic forms the cyst 

 protects the organism during its passage from one host to another, the 

 encysted form being known as the infective stage. Amongst the Sporozoa 

 it usually happens that a period of asexual reproduction is followed by 

 one in which sexual forms are developed. The appearance of these is 

 generally supposed to be an indication that unfavourable changes are 

 taking place in the environment, and that encystment, which occurs in 

 association with conjugation and the production of the zygote, is necessary. 



Another feature characteristic of many parasitic forms is the difference 

 in environment associated with different stages of development. Thus, 

 in the case of malarial parasites the human blood supplies the conditions 

 necessary for asexual reproduction and the production of gametocytes. 

 In the body of the mosquito all asexual stages quickly perish, while the 

 gametocytes continue their development, which was arrested in the human 

 blood. The sporozoites ultimately produced will develop no further in 

 the mosquito, but with the change brought about by their injection into 

 man further progress occurs. Similarly in the case of trypanosomes the 

 forms taken up from the blood by the transmitting host quickly lose 

 their power of developing in the blood, though they do so in the body 

 of the invertebrate. The metacyclic trypanosomes which are eventually 

 produced have regained the power of development in the blood. 



As a result of abundance of nourishment in the medium the cytoplasm 

 may become charged with globules of food-reserve material which appear 

 to be far in excess of that actually required. Thus, the ciVmie Balantidium 

 coli may be packed with such substances. In many cases this has 

 apparently little effect on the vitality of the organism, though it has 

 been shown that in certain forms degenerative changes result. 



A feature of this over-nourishment may be seen in certain cases of 

 gigantism. Thus Tricliomonas vaginalis is often very much larger than 

 Trichomonas hominis of the intestine. If, however, both these organisms 

 are cultivated in the same medium, the forms which appear are exactly 

 alike, so that it would seem that the large size of T. vaginalis is merely 

 an indication of overgrowth. Similarly the giant forms of Herpetotnonas 

 mirabilis, which occur in the Malpighian tubes of certain flies, can probably 

 be accounted for in similar manner. 



INFLUENCE OF SYNGAMY.— As already remarked. Protozoa which 

 become degenerate or pass into a state of depression may recover if 

 conditions of life become favourable. It is supposed that a similar 

 recovery may result from the process of syngamy. In the majority of 

 Protozoa, however, syngamy is not known to occur. In many cases 

 this is undoubtedly due to the fact that the complete life-history has not 



