THE WONDERS OF LIFE 



radial needles of these radiolaria like a crown. They 

 are found in their outer gelatine envelope, the calymma, 

 and by their contraction extend it, and so lessen the 

 specific gravity. 



Many of the aquatic protophyta and protozoa have 

 the power of autonomous and independent locomotion, 

 and this often has the appearance of being voluntary. 

 Among the simplest fresh-water protozoa are the arcel- 

 lina or thecolobosa {difjittgia, arcella), little rhizopods 

 that are distinguished from the naked amoebae by the 

 possession of a firm envelope. They usually creep about 

 in the slime at the bottom, but in certain circumstances 

 rise to the surface of the water. As Wilhelm Engel- 

 mann has shown, they accomplish this hydrostatic move- 

 ment by means of a small vesicle of carbonic acid, which 

 expands their unicellular body like an air-balloon; the 

 specific weight of the cell-body, which is of itself heavier 

 than water, is sufficiently lowered by this. The same 

 method is followed by the pretty radiolaria which live 

 floating (as plankton) at various depths of the sea. 

 Their unicellular (originally globular) body is divided 

 by a membrane into a firm inner central capsule and a 

 soft outer gelatine covering. The latter, known as the 

 calymma, is traversed by a number of water- vesicles or 

 vacuoles. As a result of an osmotic process, carbonic 

 acid may be secreted or pure water (without the salt of 

 the sea-water) be imbibed in these vacuoles; by this 

 means the specific gravity of the cell is lessened, and it 

 rises to the surface. When it desires to make itself 

 heavier and sink, the vacuoles discharge their lighter 

 contents. These hydrostatic movements of the radio- 

 laria (for which the myophrisca, still more complicated 

 structures, have been developed in the acantharia) at- 

 tain by simple means the same end that is accomplished 

 in the siphonophora and fishes by air-filled and volun- 

 tarily contractile swimming-bladders. 



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