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 weU 

 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 Eeticularia present themselves, and their importance 

 in building up large masses of chalk, limestone, etc., 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, 



Fig. 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'marveUous variety 

 of ways by which the new chambers are added which 



