i PHYLUM PROTOZOA 19 



as Amoeba, in which the entire animal consists throughout 

 life of a single cell, are distinguished as unicellular from the 

 multicellular forms in which a number of cells are combined. 

 The whole of the great group or phylum of animals the 

 Protozoa to which Amoeba belongs, are distinguished 

 by their unicellular character from all the remaining groups 

 of the animal kingdom the Metazoa. 



Among the Protozoa a large number resemble Amoeba 

 in the possession of pseudopodia or processes of the 

 protoplasm. The pseudopodia-bearing Protozoa constitute 

 one of the great divisions or classes into which the Protozoa 

 are divided by zoologists the class known as the Rhizo- 

 poda. In only a comparatively small proportion of the 

 members of this class have the pseudopodia the compara- 

 tively short and blunt shape which they have in Amoeba. 

 All the Rhizopoda with comparatively short and thick 

 pseudopodia are grouped together to form one of the 

 leading divisions or orders of Rhizopoda the order Lobosa. 

 Amoeba is one of the simplest of these. The largest among 

 the near relatives of Amoeba is Pelomyxa^ which may be as 

 much as 8 mm. in diameter, so that it is readily visible to 

 the naked eye ; its pseudopodia are very short and broad, 

 and, instead of a single nucleus, it contains a large number, 

 as well as many contractile vacuoles. Other Lobosa differ 

 from Amoeba in the presence of a shell or test enclosing 

 the protoplasm. One of these is Difflugia (Fig. 3, D\ 

 which is very common in fresh water. Difflugia has a flask- 

 shaped test formed of agglutinated sand-grains and other 

 foreign particles. The main bulk of the protoplasm is 

 contained in the interior of the shell, but comparatively 

 long pseudopodia are capable of being pushed out through 

 the mouth of the flask. An even commoner member of 

 the group is Arcella (Fig. 3, C). Arcella has a shell much 



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