22 PROTOZOA [CH. II 



sexual process. Germs which unite with one another are termed 

 gametes. 



The Protozoon Gromia possesses a thin membranous shell, 

 shaped somewhat like that of Difflugia : but the animal shows two 

 important differences ; first, the protoplasm of which the body is 

 composed, besides filling the shell, extends in a thin layer all over 

 its outer surface (2, Fig. 4), and secondly, the pseudopodia, which 

 are given off from this layer, are thin and delicate threads which 

 join and interlace with each other so as form a network. Gromia 

 seizes its prey by entangling it in these fine pseudopodia; these 

 then flow together and form a little island of protoplasm surround- 

 ing the captive, which is thus digested quite outside the main part 

 of the body ; the products of digestion being carried along the 

 pseudopodia into the protoplasm which is inside the shell. 



"We may next consider a rather larger Protozoon allied to 

 Gromia and like it possessing a shell, which however is composed 

 not of chitin but of calcium carbonate. The name of this animal 

 is Polystomella (Fig. 5). Like Gromia it possesses delicate inter- 

 woven pseudopodia which spring from the whole surface, since 

 there is a thin layer of protoplasm covering the outside of the 

 shell as well as the main mass inside it. Unlike Gromia, however, 

 Polystomella has a shell which is perforated by a large number of 

 minute holes, through which pass cords of protoplasm, connecting 

 the inner and outer parts of the animal. Polystomella is therefore 

 a typical example of the Foraminifera (Lat. foramen, a hole; 

 fero, to carry), a class which includes countless varieties of micro- 

 scopic shells, generally composed of calcium carbonate, less frequently 

 of flint (silica). The Foraminifera show essentially the same method 

 of reproduction as Arcella. For this reason, as well as because 

 many of the genera classed by naturalists as Foraminifera have 

 imperforate shells, the name Thalamophora is preferred, which 

 includes Arcella and Difflugia as well as the so-called Foraminifera. 

 The differences of structure between Polystomella, Difflugia and 

 Arcella implied in the fact that the two latter have no layer of 

 protoplasm outside the shell and that their pseudopodia are blunt 

 are considered to be of less importance than the possession of a 

 skeleton and a chromidium. In order to distinguish them from the 

 Thalamophora the various varieties of Amoeba are called Amoebidea. 

 If we examine the shell of Polystomella with the low power of a 

 microscope, we shall see that it is shaped like a rather flat snail 

 shell or the shell of the Pearly Nautilus. If, however, we dissolve 



