CHAPTEE XII. 



UNICELLULAR ANIMALS (Protozoa). 



A. Amoeba. 



(The Proteus Animalcule.) 



General Account. Amoeba is a minute organism occasionally 

 found in stagnant water, in the sediment at the bottom of ponds 

 and ditches, on the surface of water-plants, in damp earth, in 

 organic infusions of various kinds almost anywhere, in short, in 



O v 



the presence of moisture, organic matter, and other favorable 

 conditions. There are many species of Amoeba, some living in 

 salt water, others in fresh. One of the largest and commonest 

 fresh-water forms is Amoeba Proteus, which forms the subject 

 of this account.* 



Amoeba occurs in an active or motile state, and a quiescent or 

 encysted state. When active the body consists (Fig. S-i) of a 

 minute naked mass of protoplasm which in the case of large 

 specimens is barely visible to the naked eye i.e., half a milli- 

 metre (-fa inch) or less in length. This mass creeps, or rather 

 flows, actively about by the continual protrusion of lobes or proc- 

 esses of its own substance, known as pseudopodia. These may 

 be put forth from any part of the surface and again merged into 

 the general mass ; the body therefore continually changes its 

 shape, and hence the name "Proteus.' 



When the body is well extended the protoplasm is seen to 

 consist of a clear peripheral substance, the ectoplasm, and a cen- 

 tral substance, the entoplasm, filled with coarse granules which 

 give the body a highly characteristic granular appearance some- 

 times described as a "gray color.' Within the ectoplasm the 

 more fluid entoplasm freely flows, as if confined in a tube or 



* Other common forms are the smaller A. radiosa and A. verrucosa. The 

 large A. (Pelomyxa) villosa and A (Dinamcebd) mirabilis are not infrequent. 

 See Leidy, Fresh-water Rhizopods of North America. 



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