POLYMORPHISM AND LIFE-CYCLES 177 



B. LIFE-CYCLES. 



In the foregoing section the various forms have been described 

 under which one and the same species of Protozoon may occur in 

 the course of its life-history, and in response to the conditions of its 

 particular mode of life. In some species it has been seen that the 

 changes of form and structure are so slight that the species are 

 practically monomorphic, in the sense that they can be identified 

 without difficulty in any active phase of life ; no species is absolutely 

 monomorphic, since, in addition to resting states, differences in size 

 due to growth, at least, will always be found. Other species, on 

 the other hand, are polymorphic to such an extent that their specific 

 identity in different phases can only be determined by tracing their 

 development in a continuous sequence ; and in extreme cases of 

 polymorphism the life-history becomes a varied pageant of dis- 

 similar forms succeeding each other in more or less regular order, 

 determined largely, if not entirely, by the conditions of the environ- 

 ment. In a former chapter the distinction has been drawn between 

 a developmental cycle, consisting of a recurrent series of different 

 forms, and the complete life-cycle, consisting of the whole series 

 of forms or phases which appear between one act of syngamy and 

 the next. The complete life-cycle may comprise many develop- 

 mental cycles. 



As a concrete example of a lif e-cycle comprising a great number of 

 different forms, and in which also the development may follow more 

 than one course, the life-cycle of Arcella vulgaris may be selected 

 (Fig. 80). The life-history of this form has now been made known 

 in detail by the combined labours of many investigators, amongst 

 whom Hertwig (65), Elpatiewsky (144), Swarczewsky (101), and 

 Khainsky (145), must be specially mentioned. 



The form which may be taken as the starting-point of the life- 

 cycle is a minute, amoeba-like form, with a single nucleus (Fig. SO, A). 

 The amoebula, when set free, feeds, grows, and becomes after a 

 time spherical in form with radiate pseudopodia (Fig. 80, B) ; in 

 this stage it resembles a species of the genus Nuclearia. After a time 

 the Nudearia-foim secretes a shell, and now resembles an example 

 of the genus Pseudochlamys (Fig. 80, C). With further growth, 

 chromidia are given off from the nucleus into the cytoplasm, the 

 nucleus divides into two, and the animal thus assumes gradually the 

 characters of the adult Arcella (Fig. 32 ; Fig. 80, D). It has a 

 chitinous shell, circular in outline, flattened in profile-view, and 

 slightly concave on the under-side, in the centre of which is a large 

 circular aperture through which the pseudopodia stream out. The 

 body-protoplasm contains two nuclei situated approximately at 



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