I PHYLUM PROTOZOA 19 



as Amceba, in which the entire animal consists throughout 

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

 multicellular form in which a number of cells are combined. 

 The whole of the great group or phylum of animals — the 

 Protozoa — to which Amceba belongs, are distinguished from 

 all the remaining groups of the animal kingdom — the 

 Metazoa — by their unicellular character. 



Among the Protozoa a large number resemble Amceba in 

 the possession of pseudopodiaor 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 Rhizopoda. In only a 

 comparatively small proportion of the members of this class 

 have the pseudopodia the comparatively short and blunt 

 shape which they have in Amceba. 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. Amceba is one of the 

 simplest of these. The largest among the near relatives of 

 Amceba 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 Amceba in 

 the presence of a shell or test enclosing the protoplasm. 

 One of these is Dijjiugia (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. It pierces 

 the wall of the cells of Spirogyra, inserts its pseudopods, lift- 

 ing the entire cell-contents out and passing them into its 



