78 THE SMALLEST LIVING THINGS 



times, the last-formed skeleton enclosing those which were 

 formed before. In this way six or more concentric, attached, 

 latticed spheres like Chinese boxes are formed by some types of 

 Radiolaria and remain as a permanent record of the activity of 

 these minute organisms. All Radiolaria, however, do not pre- 

 cipitate silica; some of them manufacture a horn-like compound 

 made of strontium sulphate, with quite a different, but always a 

 complex type of skeleton. 



Limestone shells of Foraminifera and skeletons of Radio- 

 laria are constantly raining onto the sea bottom and enormous 

 beds of foraminiferal and radiolarial ooze have accumulated 

 there. Such beds have been cast up from time to time in the 

 geologic history of the earth and have given rise to great tracts 

 of limestone rock, as in the chalk cliffs of England, or to silicious 

 earth, such as that composing the Barbadoes and other islands of 

 the West Indies. 



Reproduction Among the Sarcodina 



In structure, the rhizopod Amoeba proteus, with its 

 pseudopodia forming at any point, is one of the simplest types of 

 protozoa, and its constantly changing shape has always been a 

 fascination to the microscopist. It reproduces by cell division 

 after the pseudopodia are drawn in and waste and foreign mat- 

 ters have been extruded. Division becomes a bit more compli- 

 cated in shell-bearing types. Here, preparatory to division, the 

 protoplasmic body swells by absorption of water and a large por- 

 tion of the protoplasm protrudes from the shell mouth. This 

 bud assumes the form of the parent, secretes a membrane of 

 chitin, develops pseudopodia, and, after the parent nucleus has 

 divided and provided the bud with one, breaks away from the 

 parent organism. It is a process of budding division analogous 

 to that in diatoms and desmids. 



In Foraminifera, when the organism is fully developed, its 

 protoplasm breaks up into a multitude of minute ameba-like cells, 

 a process to which the term sporulation is applied. Each of 

 these spores settles down after a short period of swarming and 

 develops a new organism similar to the parent with the exception 

 that the first shell chamber formed is larger than the first chamber 

 of the parent organism. Thus two types of individuals arise, 



