78 PROTOPLASM OF PROTOZOA 



and decrease at the same time. In the Radiolaria periodic migrations take 

 place to and from the surface layers of the sea; these are brought about 

 by changes in the vacuolar contents of the hydrostatic layer, which, ac- 

 cording to Brandt (1885), is lighter than water. Schewiakoff (1927) 

 has described the presence of a clearly defined gelatinous hydrostatic 

 layer in the Acantharia (Radiolaria). Although the specific gravity of 

 Noctiluca (1.014) is less than that of sea water (1.026) in which it 

 floats, according to Massart (1893), E. B. Harvey (1917) has shown 

 that this form can lessen and increase its specific gravity in a regulatory 

 fashion. Lund and Logan (1925) have shown that the increase in specific 

 gravity, following strong mechanical shock or electrical stimulus, is 

 caused by the coalescence of large vacuoles and the liberation of their 

 contents, which diffuse through the pellicle. The density of the solution 

 in the vacuoles is less than that of sea water; according to Ludwig 

 (1928b) this is because of its lower salt content, its osmotic pressure 

 being about half that of sea water. Most marine animals possess body 

 fluids almost isotonic with the external fluid, but Noctiluca, together with 

 the marine teleosts, are hypotonic and, to a great extent, osmotically 

 independent. Such organisms must be impermeable to water, absorb 

 water in some way without salts, or take in sea water and excrete salts. 

 Marine teleosts apparently take in sea water and excrete the excessive 

 salts, and, since the membrane of Noctiluca is permeable to water, os- 

 motic work (negative osmotic force) must be done by the membrane 

 (E. B. Harvey, 1917) in maintaining this steady state instead of os- 

 motic equilibrium. 



The protoplasm of marine Protozoa frequently becomes much vacu- 

 olated upon transfer to fresh water. The marine variety of Actinophrys 

 sol, according to Gruber (1889), has thick, granular protoplasm poor 

 in vacuoles and entirely lacking a contractile vacuole; during gradual 

 transfer to fresh water the protoplasm becomes foamy with bubbles and 

 a contractile vacuole appears, so that the organism is indistinguishable 

 from the fresh-water variety. The formation of vacuoles and the entrance 

 of water into them undoubtedly lowers the specific gravity in this form, 

 when transferred from salt to fresh water. The spine-like pseudopodia 

 of Heliozoa, Radiolaria, and other floating forms also serve as a protec- 

 tion against sinking. 



The first estimation of the specific gravity of a protozoan apparently 



