26 MANUAL OF ZOOLOGY SECT. 



geological periods the beds of chalk of the Cretaceous 

 period consisting, like the Globigerina ooze, in great measure 

 of the shells of Foraminifera, though apparently not 

 formed under the same conditions of depth. Another case 

 of massive deposition of Foraminifera in a former geological 

 period is the Nummulitic Limestone, a bed of limestone 

 made up for the most part of the shells of comparatively 

 gigantic Foraminifera, the Nummulites (Fig. 5, n). 



A Rhizopod by no means uncommon in fresh water is the 

 so-called Sun-Animalcule, Actinophrys sol. The body of 



FIG. 6. Actinophrys SOl. a. axial filaments of pseudopods ; n. nucleus ; 

 p. pseudopod. (From Lang's Comparative Anatomy, after Grenacher.) 



Actinophrys (Fig. 6) is nearly spherical, and contains a 

 large nucleus and numerous vacuoles, some of which, 

 situated near the surface, are contractile. The most 

 characteristic feature is formed by the pseudopodia, which 

 instead of being comparatively short and thick, as in 

 Amoeba and in the other Lobosa, or extremely delicate, 

 flexible and thread-like, as in the Foraminifera are slender, 

 but comparatively stiff, and stand out straight from the sur- 

 face of the sphere in a radiating manner ; they are capable 

 of only very slow movements. The pseudopodia owe their 

 stiffness to the presence of a rod of chitinoid material 



