PROTOZOA 13 
ules may be seen streaming, evidence of the active movement 
of the protoplasm. They are never entirely naked, but are 
enclosed in a shell, which may be chitinous, calcareous, or com- 
posed of agglutinated sand grains (Fig. 5). There may be one 
or many nuclei, and a contractile vacuole has not been observed 
in most cases. Their method of reproduction is not very well 
known; it may take place by fission, or by the formation of 
buds. There are both marine and freshwater representatives, of 
this class. The enormous variety of forms under which the 
shells of the Reticularia present themselves, and their importance 
in building up large masses of chalk, limestone, ete., have always 
attracted the attention of naturalists. The class was formerly 
divided into two groups: the Perforata, those whose shell is 
pierced by numerous fine pores all over its surface, through 
which the filiform pseudopodia find exit ; and the Imperforata, 
Fic. 6.—Globigerina, as cap- 
tured in the tow-net near the 
surface. 
without the minute pores, but with one or more larger openings, 
for the exit of the protoplasm. This division is, however, tending 
to be obliterated. Many of the shells consist of one chamber 
only (monothalamia, Fig. 5), others, as they grow in size, accom- 
modate their increased bulk by the addition of more chambers 
(polythalamia, Fig. 7), and it is chiefly the marvellous variety 
of ways by which the new chambers are added which 
