128 



THE RISE OF ANIMAL LIFE 



Fig. 7-22. This amoeba-like protozoan {Arcella) carries 

 around a "house" into which it may retreat when 

 its life is endangered. The shell is brown in color 

 and, because the opening through which the animal 

 passes lies in the center, it resembles a doughnut. 

 Note the long pseudopods protruding out from the 

 shell. This is a plastic model. 



derous attack of parasites of all kinds, rather 

 than any fundamental inability of the peo- 

 ple themselves to thrive. 



Class Sarcodina 



Although members of this class resemble 

 the amoeba to some extent, there is wide 

 variation in form and structure in the group 

 as a whole. Among the fresh-water forms 

 there are those, such as Diffliigia (Fig. 7- 

 21), that build houses for themselves. This 

 tiny animal gathers grains of sand and ce- 

 ments them together to form a pear-shaped 

 outer covering into which it may retreat 

 when in danger. PJagiophrys and Arcella 

 ( Figs. 7-21 and 7-22), likewise, build houses 

 for themselves, but in this case they are se- 

 creted by the animals. When observed 

 through the microscope, the shell of Arcella 

 resembles a doughnut. Corresponding to the 

 hole in the doughnut is the opening through 



which the amoeboid form passes as it re- 

 tracts or extends itself from the shell. 



Another fresh-water sarcodinid of inter- 

 est is the "sun animalcule," Actinophnjs sol 

 (Fig. 7-21), which resembles a miniature 

 sun when it is floating in the water. The 

 radiating, ray-like pseudopods seem to have 

 a paralyzing effect upon other Protozoa, 

 such as euglena, which serve as a food 

 source. There are many related species of 

 this spectacular protozoan. Together they 

 constitute the order Heliozoa, a group that 

 is commonly found in the oceans of the 

 world. One, Oxnerella (Fig. 7-23), is a par- 

 ticularly beautiful heliozoan. 



A large group of forms closely resembling 

 Heliozoa form the order Radiolaria. These 

 are also distributed throusihout the oceans 

 of the world and float near the surface of 

 the water. Most of them possess a siliceous 

 skeleton which sinks to the ocean floor 

 when the animal dies, forming a thick, 

 mucky layer called "radiolarian ooze." This 

 is particularly extensive in the Pacific and 

 Indian Oceans. Skeletons of these animals 

 are also found in rocks and have been used 

 by geologists in learning about the history 

 of the earth. 



Marine sarcodinids that have even greater 

 significance to the geologists are found in 

 the order Foraminifera, which secrete shells 

 of almost pure calcium carbonate. While 

 some, such as Boderia (Fig 7-23), possess 

 extremely thin, delicate outer coverings of 

 this substance, most of them secrete a heavy 

 shell, for example, Discorhis and Peneroplis 

 (Fig. 7-23). These many-chambered, snail- 

 like shells, are produced as the animals grow 

 larger and are perforated with tiny holes 

 through which the fine pseudopods project. 

 When they die their skeletons, like those of 

 the Radiolarians, form a "globigerina ooze" 

 ( named after the most dominant form, Glo- 

 bigerina). This eventually becomes chalk 

 many hundreds of feet thick. The Cliffs of 

 Dover are an outstanding example of this 

 phenomenon. Wherever this chalk appears 

 on land, one can be certain that it was once 



