PROTOZOA RIIIZOPODA. 151 



important portion of the earth's crust, is now known to 

 be almost wholly composed of a single species of Fora- 

 minifera (Globigerina). Several other limestones also 

 have been mainly formed by them. The earliest vestige 

 of animal life found in the rocks of the earth the eozoon 

 of the Lawrentian strata is a foraminifer. 



The modern forms are all microscopic, but the extinct 

 nummulite (Lat. nummus, money ; Gr. lithos, a stone) 

 which has formed large masses of limestone, extending 

 from France to China, is sometimes three inches in cir- 

 cumference. The eozoon seems also, by continuous 

 budding, to have attained a large size. 



217. (3.) The Radiolaria (Lat. radius, a ray) form the 

 third group of Rhizopoda. They are so called because 

 the pseudopodia surround the body like rays. They are 

 distinguished from the Foraminifera by having a siliceous 

 instead of a calcareous skeleton, which sometimes takes 

 the form of a shell ; and, in other cases, consists of loose 

 spicula (Lat. spiculum, a point), or needles. The best 

 known members of this group are the Polycystina (Gr. 

 2}olus } many; kustis, a bladder), which are even smaller 

 than the Foraminifera. They are furnished with minute 

 glassy shells, which, under the microscope, are objects of 

 extreme beauty. The animal consists of a little mass of 

 brown sarcode, which does not completely fill the shell. 

 The pseudopodia are protruded through apertures in the 

 shell, but they do not form a network. The white mud 

 dredged up from the bottom of the Atlantic contains a 

 considerable proportion of these shells, mixed with those 

 of the Foraminifera. They are also found very abun- 

 dantly as fossils in some tertiary rocks. 



218. The Thalassicollida (Gr. thalassa, the sea ; kolla, 

 glue) are gelatinous animals found floating near the sur- 

 face of the ocean. They are sometimes an inch in 

 diameter. Their bodies are surrounded by radiating 

 pseudopodia, which sometimes form a network. The 

 skeleton consists of a series of flinty needles scattered 

 through the sarcode. 



