36 THE INVERTEBRATA 



which have no meganucleus have in their nuclei trophochromatin 

 which is destroyed at syngamy, this conclusion may be extended to 

 them also. 



(2) Variety in a protozoan species is of three kinds : (a) that which 

 results from the production of different combinations of genes at 

 syngamy, and is permanent, forming races {pure lines) like those 

 which exist in higher organisms in the absence of cross-fertili- 

 zation; such pure lines have, for instance, been found in respect 

 of body-length in cultures of Paramecium, each line in the culture 

 breeding true so long as asexual reproduction continues; (b) that 

 which results from the spontaneous appearance of mutations ; this also 

 is permanent; it has been studied in Ceratium and other genera; 

 (c) that which results from modification of the individual by the direct 

 action of the environment; this, like mutation, produces differences 

 between individuals of a pure line, but it is not permanent, though it 

 may be inherited for several generations before it disappears. It 

 would seem that, apart from the occasional appearance of mutations, 

 the permanent varieties in a species are produced only by syngamy. 



Here may be mentioned the union of individuals by fusion of 

 their cytoplasm, the nuclei remaining distinct, which is practised by 

 the Mycetozoa (Fig. 73 F) and in some other cases. This process, 

 which is not syngamy, is known as plastogamy, and its product as a 

 Plasmodium. 



The life of a protozoon passes in the course of generations through 

 a cycle in which individuals of different kinds succeed one another. 

 The life cycles of various protozoa differ greatly, being related to the 

 vicissitudes of the environment of the species and to the need for 

 distribution as well as to the recurrence at intervals of conjugation, 

 but it is possible to formulate a type of which all of them may be 

 regarded as variants. After a period of "vegetative" existence and 

 increase by asexual reproduction, during which the individuals are 

 known as agamonts, there appears a generation known as gamonts 

 because they produce gametes, the latter unite in pairs, and the zygote 

 or sporont gives rise to a generation of sporozoites which, becoming 

 agamonts, repeat the asexual part of the cycle. The table on p. 37 

 shows this typical life history. 



In comparing this table with the actual course of the cycle in any 

 species, the student should remember: 



(i) that in each part of the cycle fission may take place in any of the 

 modes described above, and that the agamogony of a species may 

 proceed in more than one of these ways (as, e.g., that of Amoeba 

 proteus by binary or multiple fission) ; 



(2) that in the cycle of most protozoa there is a point at which ad- 

 justment must be made to unfavourable conditions, either recurring 



