I PHYLUM PROTOZOA 21 



The bulk of the protoplasm is, as in Difflugia, enclosed 

 within the test, but a considerable portion of it may be 

 pushed out in the form of pseudopods. Several nuclei and 

 a contractile vacuole are contained in the protoplasm. The 

 body of the animal is colourless, and is attached to its test, 

 says Stokes, " by fine threads of its own substance." There 

 are several species in our fresh-water pools, among them 

 Arcella vulgaris (Fig. 3, C). 



All the rest of the Rhizopoda differ from the Lobosa in 

 having the pseudopodia in the shape of slender threads. 

 Of these a remarkable and interesting group is the order 

 Foraminifera. A Foraminifer has a shell which is nearly 

 always composed of carbonate of lime. This we can readily 

 demonstrate by placing a drop of hydrochloric or nitric acid 

 on a mass of the shells, when they dissolve with efferves- 

 cence. In some Foraminifera the shell has a wide opening 

 on the exterior as in Difflugia and Arcella ; in others there 

 is no large opening, but the wall of the shell is perforated 

 by a number of minute pores scattered over its surface. 

 The greater part of the protoplasm is enclosed within the 

 shell, but part of it (Fig. 4) streams out from the single 

 large opening, or from the pores, in the form of slender 

 thread-like radiating pseudopodia, which, when they come 

 in contact with one another, may coalesce, and may in this 

 way give rise to a network. The protoplasm in the interior 

 contains a nucleus, but no contractile vacuole. The shape 

 of the shell is sometimes spherical, sometimes flask-shaped, 

 sometimes oval or elliptical. Only in a comparatively small 

 number of Foraminifera does it remain simple (/, 2) ; in the 

 great majority, though the shell when first formed is simple, 

 a little process or bud of protoplasm soon projects through 

 the wide opening or through the pores; this increases in 

 size, and becomes enclosed in a shell like the original one, 



