148 



SYSTEMATIC ZOOLOGY. 



move about and obtain their food. The subdivisions of the 

 class are based primarily upon the character of the pseudo- 

 podia. 



Order I. Lobosa. With broad pseudopodia few in 

 number. Best known is the genus Amceba of fresh water. 

 Other genera have shells (fig. 3). 



' 



FIG. 3. Difflugia. A lobose FIG. 4. Foraminifera. A, Biloculina with 



Rhizopod with a shell of extended pseudopodia (after Schultze); B, 



grains of sand. Common chambered shell of Globigerina. 

 in fresh water. 



Order II. Foraminifera. Numerous slender pseudopo- 

 dia which branch and unite to form networks. Nearly 

 all are marine. Most species form calcareous shells which 

 in many species increase with growth by adding addi- 

 tional chambers to the original one. There is a terminal 

 aperture for the extension of the pseudopodia and in many 

 minute pores through the sides of the shell. These forms 

 occur in large numbers in certain seas and the dead shells 

 are forming thick layers at the bottom of the ocean. They 

 have done the same in ages past and in various parts of 

 the earth are thick beds of limestone largely built up from 

 their dead shells. Indeed they are the largest contributors 

 to the formation of rock of all animals. 



