26 



MANUAL OF ZOOLOGY 



From the name of the genus — Globigerina (Fig. 5, 6) — 

 which occurs in the greatest abundance in this deposit, it is 

 known as the Globigerina ooze. In the deepest parts of the 

 ocean the Globigerina ooze is entirely absent, the calcareous 

 shells of the Foraminifera apparently becoming entirely dis- 

 solved before they can reach such great depths. It is inter- 

 esting to note that similar deposits were formed in previous 

 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, 11). 



A Rhizopod by no means uncommon in fresh water is 

 the so-called sun-animalcule, Actinophrys sol. The body 



Fig. 6. — Actinophrys sol. a, axial filaments of pseudopods; «, nucleus; 

 p, pseudopod. (From Lang's Comparative Anatomy, after Greenacher.J 



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

 large nucleus and numerous vacuoles, some of which, situ- 

 ated near the surface, are contractile. The most charac- 

 teristic feature is formed by the pseudopodia, which, instead 



