AMCEBA 



The Protozoa are structurally simpler than other animals in 

 that their bodies are not divided into cells ; each individual con- 

 sists of a mass of protoplasm with some sort of boundary layer and 

 containing a single nucleus, or occasionally two nuclei or more. 

 We shall begin with the genus Amceha, not because it is really 

 the simplest of Protozoa but because it is easy to observe. There 

 are several species, which agree in a number of points. The 

 protoplasm is divided into a clear outer ectoplasm and an inner 

 granular endoplasm ; the surface is specialised as a thin 

 plasmalemma, of dough-like consistency, but there is no secreted 

 cell -wall ; and from the surface there are put out from time to 

 time one or more blunt finger-like processes called pseudopodia. 

 One of the largest species is Amceha proteus, which is about one- 

 hundredth of an inch across and lives in the mud of ponds, 

 though it is not very common. A. lescherce (Fig. 9) is larger and 

 A. discoides (Fig. 11) smaller, but they are otherwise difficult for 

 the student to distinguish. The following account applies, unless 

 otherwise stated, to all three species. A. proteus produces 

 about three or four pseudopodia at a time, all of which are of 

 relatively large size and sub-cylindrical in shape. There is normally 

 one nucleus, which is more or less centrally placed, but about 

 five per cent, of individuals have two, three, or occasionally four 

 nuclei. The nucleus is only just visible in living specimens, but 

 when the animal is killed and stained with certain dyes, the 

 nucleus is conspicuous, since it takes up the colour more deeply 

 than the cytoplasm, and can be seen to be lens-shaped. Just 

 behind the nucleus is a clear spherical space, the contractile 

 vacuole. Besides the granules, the endoplasm contains several 

 small animals or plants which have been taken in as food. 



MOVEMENTS 



It is common, but incorrect, to describe Amoeba as shapeless ; 

 in fact each species has a characteristic form, by which it can be 

 recognised, though it is true that there is more variation in that 



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