XII 



BACKBONELESS ANIMALS 



223 



simplest Protozoa, such as Protomyxa, exhibit a cycle of 

 amoeboid, encysted, and flagellate phases, not having 

 taken a decisive step along any one of the three great 

 paths. Moreover, the cells of higher animals may be 



FIG. 62. PROTOMYXA. 



1, Encysted ; 2, dividing into many units ; 3, these escaping as flagel- 

 iate cells ; 4, sinking into an amreboid phase ; 5, fusing into a plasmodium. 



(From Chambers's Encyclop. ; after Haeckel.) 



classified in the same way. The ciliated cells of the 

 windpipe or the mobile spermatozoa correspond to 

 Infusorians ; mature ova, fat-cells, degenerate muscle- 

 cells, correspond to Gregarines ; white blood-corpuscles 

 and many young ova are amoeboid. 



Relation to the Earth. The floor of the sea for a variable 

 number of miles (not exceeding 300) from the shore is 

 covered with a heterogeneous deposit, washed in great 

 part from the nearest continent. In this deposit shells 

 of Foraminifera usually occur, but they become more 

 numerous farther from the land, where the floor of the 

 sea is often covered with a whitish " ooze," consisting in 

 the main of Foraminifera which in dying have sunk from 

 the surface to the bottom. They are forming the chalk 

 of a possible future, just as many chalk-cliffs and pure 

 limestones represent the ooze of a distant past. In 

 other regions Radiolarians (flinty Rhizopods) or Diatoms 

 (small plants) or Pteropods (minute molluscs) are very 

 abundant. As the Foraminifers have made much of the 

 chalk, so Radiolarians have formed less important siliceous 

 deposits, such as the Barbados Earth, from which Ehren- 

 berg described no fewer than 278 species. At marine 

 depths greater than 2,500 fathoms the Globigerina or 

 other Foraminifer shells are no longer present, not because 

 there are none at the surface, but apparently because 



