Marvels of the Universe 



73 



rigid, and unaffected by anj- chemical agents except those which are known solvents of flint ; 

 whereas the shells of the Foraminifera are composed of carbonate of lime. Occasionally flinty 

 Foraminifera have been found, but these have been changed. Their original composition was 

 carbonate of lime, but owing to the action of water containing silica, the chalky material was 

 replaced by flinty matter. 



There is still another important point of difference between these two great groups of marine 

 organisms. The Foraminifera are divided into two sub-families — the Perforate and the Imper- 

 forate. But in the Polycystina there is no distinction afforded by Perforate or Imperforate, since 

 all the Polycystina are Perforate. There is no exception to the rule that the flinty skeleton is 

 so pierced b}- apertures in every direction and in every portion — excepting the spinous projections, 



POLYCiSTlNS. 



This illustration shows how the Polycystins occur in Nature, the various forms mixed up together 

 would represent a mere speck of dust before they were magnified. 



The entire group shown 



which are so but rarely — that the most complete communication may be said to exist between the 

 contents of every- chamber and the investing film of protoplasm on the exterior. 



The flinty skeletons of the Polycystins are of many and curious shapes. Some are like 

 baskets : others are spheres covered with spikes ; others are cone-shaped ; others seem as if they 

 were wTeathed into crowns ; others are pyramidal, while many of them resemble the famous ivory 

 baU-within-baU ornaments fashioned by the skilled Chinese artificer. 



All of them, however small, are latticed. Even those whose diameter is the hundred and 

 fiftieth part of an inch have their " windows," or apertures, where it would seem impossible for 

 such to exist. 



In those specimens which consist of concentric balls there are rods of flint supporting each 

 sphere in its place, so that the complete structure is very strong. 



In certain of the Polycystins the jelly-like body ramifies into all parts of the internal spaces 

 as well as the external parts of the flinty skeleton. In others the creatures can retreat into the 

 upper domes of the skeletons. Spikes or spines project at regular intervals from the surface. In 



