I PHYLUM PROTOZOA 21 



pushed out in the form of pseudopods. Several nuclei and 

 a contractile vacuole are contained in the protoplasm. 



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 (Fig. 5, i, 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 in- 

 creases in size, and becomes enclosed in a shell like the 

 original one, but usually a size larger, remaining in firm 

 connection with it and the cavities of the two communicating 

 with one another through the original opening or openings 

 at which the bud first appeared. From this second shell, in 

 turn, a bud is given off in the same manner, and the process 

 is repeated again and again, until, instead of a single particle 



