8 THE PROTOZOA THE DAWN OF LIFE 



when we come to seek for their fossil remains in the rocks, for 

 then we find that they range in geological time from the Lower 

 Cambrian strata to the present day. The beautiful chalk cliffs 

 that form a natural bulwark along many miles of the English 

 coast, and the noble North and South Downs, consist almost 

 wholly of the fossil remains of Globigerinids, while another species 

 of Foraminifera, the Nummulites, have contributed largely to the 

 composition of the great Eocene limestones, and are the chief 

 constituents of the stone that was used in the building of the 

 great Egyptian pyramids. 



The species of Foraminifera to be found between tide marks 

 on the seashore, and extending to the deep water beyond, are 

 very numerous, and may be collected from the surface of the sand 

 and mud left exposed by the receding tide, and upon the fronds of 

 the seaweeds in the pools. Some may be met with in the brackish 

 waters of estuaries ; some are only to be found at various depths 

 of the sea, or living among the bottom ooze ; while others again 

 float freely at or near the surface of the sea, and form at times a 

 considerable proportion of the surface Plankton. 1 



Such pelagic 3 forms as Globigerina, and its allies, have the 

 surface of their shells extended by extraordinarily delicate spines, 

 which not only help to prevent the organisms from sinking, but 

 also permit the protoplasm protruding from the mouth and fine 

 holes in the shell to form a curious bubbly film between them, 

 and to stretch out long, fine threads for the capture of food. On 

 the death and decay of the protoplasm of these buoyant Fora- 

 minifera, their shells gradually sink to the bottom, losing in their 

 passage downwards their more readily soluble delicate spines, 

 and at last accumulating at depths where their more solid cal- 

 careous parts form a light oozy mud termed " Globigerina-ooze." 

 At depths between 1,000 and 2,000 fathoms, on the floor of the 

 Atlantic, great beds of this Globigerina-ooze, or modern chalk, are 

 forming to-day in just the same way as the chalk cliffs of England 

 were formed on the floor of a sea during a past geological age. 



The shells of the Foraminifera are in many species composed 

 of carbonate of lime ; in one group, called the Arenaceous Fora- 

 minifera, the shells are composed of foreign particles, such as sand 



1 From a Greek word with the meaning " that which is drifted." 

 * Pelagic buoyant, floating at whatever depth found. 



