THE PHYLUM PROTOZOA 



159 



Difflugia- 



Ac t inophry S 



Globiderina. 



Figure 8.10. Other sarcodinids. Difflugia is an ameba with a shell cemented from 

 sand particles. Actinophrys is a member of the fresh-water order Heliozoa and Globi- 

 gerina belongs to the marine order Foraminifera. 



small organisms that touch their pseudopods, first engulfing them in 

 food vacuoles and then drawing them into the central mass. Although 

 the prey are obviously paralyzed upon touching the pseudopods, the 

 method of "stinging" is unknown. 



Rad'iolaria and Foraminifera. Members of the two marine orders 

 Radiolaria and Foraminifera are adapted to floating. Radiolarians re- 

 semble heliozoans but they possess an internal skeleton made of silica. 

 These glassy frameworks combine porous spheres with radiating spines 

 to produce intricate and beautiful patterns (Fig. 8.11). Silica is durable, 

 and many deep-water marine sediments are composed largely of radio- 

 larian fossils. Similar fossils are found in rocks that date from pre-Cam- 

 brian times, before fossils of metazoa occur. 



Foraminiferans secrete an external porous capsule of calcium car- 

 bonate through which pseudopods project into the water. As the animal 

 grows, it expands its home by adding new chambers. Many genera, such 

 as Globigerina (Fig. 8.10), are abundant. Their skeletons rain contin- 

 ually upon the ocean floor, and in the more shallow seas where they are 

 not dissolved they may form large deposits, such as those which now 

 comprise the chalk cliffs of Dover. While species of the genus Globi- 

 gerina add new chambers in a somewhat irregular fashion, most species 

 add them in a systematic pattern, often in a coiled sequence like a 

 snail shell. The shape and arrangement of the chambers serves to 

 identify the species. .Although the Foraminifera are prominent mem- 

 bers of the marine plankton, most of the species, especially those with 

 heavy shells, live on or near the bottom in relatively shallow water. 



During the coal age (later part of the Paleozoic era), when the 

 major coal and oil deposits were laid down, a family of foraminiferans, 

 the Nummulitidae, flourished and died. In that brief space of geologic 

 time (75 million years) thousands of species of nummulites developed, 

 most of which lived a very short time before becoming extinct. These 

 were immense protozoa, up to an inch in diameter, that lay upon the 

 bottoms of the shallow seas. Their fossils are now found in the deposits 

 that contain oil. As an oil well is drilled down into the rock, it passes. 



