ZOOLOGY 



border, and numerous food vacuoles. In addition we find one 

 or more contractile vacuoles which enclose no liquid, but a 

 gas, possibly CO^. This gas vacuole serves as a hydrostatic 

 balance ; when it disappears the ArceUa sinks. Two individuals 

 are sometimes found lying with their flat surfaces applied to 

 one another in the process of conjugating. This has been in 

 some cases observed to precede reproduction, which takes place 

 by the constriction of small portions of protoplasm, either from 



Fig. 5. — Arenaceous Foraminifera. 



a. Exterior of Saccammina. i. The same laid open. 



c. A portion of the test more highly magnified. d. FUulina. 



e. Portion of test more highly magnified. 



the protruding pseudopodia or from the protoplasm enclosed in 

 the shell. In the latter case the abstricted portions escape 

 through the shell mouth and grow up into new ArceRae. 



Diffiugia is a genus with a soft shell strengthened by the 

 presence of sand particles and diatoms ; the various species have 

 various shapes, some being flask or urn shaped, and one is 

 slightly coiled. 



Class IV. Reticularia (Foraminifera). 



In these Protozoa, the pseudopodia are filiform, and anas- 

 tomose into a fine reticulum, along the strands of which gran- 



