I2 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 Arcella 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 
Fic. 
5.—Arenaceous Moraminifera. 
a. Exterior of Saccammina. b. The same laid open. 
c. A portion of the test more highly magnified. d. Pilulina. 
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 Arcellae. 
Difflugia 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. 
Ciass IV. Reticularia (Foraminifera). 
In these Protozoa, the pseudopodia are filiform, and anas- 
tomose into a fine reticulum, along the strands of which gran- 
