230 



GENERAL PHYSIOLOGY 



maintains the undisturbed Thcdassicolla suspended at the surface 

 of the sea.' This can be made out readily by removing from the 

 living animal single constituents of the cell, by cutting off the 

 layer of jelly, isolating the vacuole-layer and extirpating the cen- 

 tral capsule with its contents. All constituents, when isolated, 

 sink to the bottom of the water, except the vacuole-mass ; this 

 remains at the surface, and, if submerged, continually returns to 

 it.^ Correspondingly, the whole ThalassicoUa begins to sink as 

 soon as the vacuole-layer collapses by the bursting of the vacuoles, 

 which takes place as a result of stimulation, in nature especially 

 from the impact of violent waves. Then the cell falls into more 

 quiet depths, and thus is protected from entire destruction ; the 



Fig. 92.— ThalassicoUa nudeata, a spherical radiolarian cell in section. In the middle of the 

 central capsule, which is surrounded by black pigment, lies the vesicular nucleus. The central 

 capsule is surrounded by the Tacuole-layer, which is enveloped by a zone of jelly and sends 

 through the latter radiating, thread-like pseudopodia. 



vacuole-layer can regenerate itself, and the ThalassicoUa, increasing 

 in volume, in quiet weather rises again from the depths to the 

 sunny surface. The great importance of this manner of move- 

 ment for the life of pelagic organisms is evident. 



It is a question how the contents of the vacuoles can become 

 lighter than the surrounding sea-water. The cause of the appear- 

 ance of vacuoles, the formation of which can easily be observed in 

 any isolated central capsule, consists in the accumulation through- 

 out the protoplasm of osmotic substances, which cause the water 

 to come in from the outside to them through the protoplasm. The 

 size of the vacuole increases in proportion as the formation and 

 concentration of osmotic substances in the protoplasm increase, for 

 an equalisation of the osmotic pressure in the liquid of the vacuole 



' GJ. Brandt ('85). 



Of. Verworn ('93). 



