- 3 — 



organic fibers and meshes, while the rudiments of an organic 

 cuticle have been detected in certain of the highest types. 

 Most significant are the calcareous and silicious intracellular 

 skeletons of the Foraminifera and radiolarians, for in these 

 we seem to have an indication of the origin of the animal 

 skeleton from primitive raphide-like inclusions, in the Fora- 

 minifera through structures comparable to the granules and 

 lime knots seen in the myxomycètes, and in the radiolarians 

 through a radial arrangement of a number of silicious needles. 

 It should be mentioned also that some of the radiolarians 

 construct their skeletons of an organic substance, acanthin, 

 allied to lignine. Only animals which possess a blood vascular 

 system combined with a respiratory system are capable of 

 developing a continuous external or internal skeleton such 

 as we see in the echinoderms, arthropods, tunicates and 

 vertebrates. 



The plants w^hich live on land require no effort whatever 

 on their part to maintain the circulation necessary to their 

 existence, which is effected through the purely physical 

 processes of evaporation and osmosis ; they are therefore able 

 to develop the inorganic external deposit within their cell 

 walls leaving only small windows for the necessary renewal 

 of their cell fluids. Air being much lighter and more mobile 

 than water, flexibility is a very essential attribute of fixed 

 air-living organisms, while the elements entering into the com- 

 position of lignine are more universally and uniformly 

 abundant and more readily available than any of the elements 

 forming the basis of the inorganic compounds suitable for 

 skeletons ; thus the plants have developed woody skeletons 

 in contrast to the corresponding animals in which the skeleton 

 is wholly, or at least in large part, inorganic. 



The plants living in the sea are confronted with more or 

 less the same problems as the marine animals, and they must 

 keep their cell walls at a minimum thickness ; they meet these 

 problems in the same way, by building up calcareous supports 

 upon which they rest or by developing a structure more or 

 less suggestive of that of some cœlenterates. 



Similarly many of the animals of this type under adverse 

 conditions, the drying up of the water in which they live, the 

 lowering of the temperature, etc., enter a resting stage or form 



(400) 



