198 THE PROTOZOA 



Substances that precipitate albumens have a retarding effect combined with 

 dilatation of the vacuole. Degen, following Hartog, regards the vacuole as 

 primarily a mode of compensation for the tendency of the protoplasm to take 

 up water by imbibition, a tendency checked or inhibited by changes in the 

 tonicity of the medium. Thus Zuelzer (222) found that Am&ba verrucosa, 

 if transferred gradually from fresh water to sea- water, lost its contractile 

 vacuoles ; at the same time its protoplasm shrank and altered in character, 

 and the nucleus acquired a different structure and appearance. When re- 

 stored to fresh water, the contractile vacuoles reappeared, and the nucleus 

 and cytoplasm became of normal character. These experiments indicate 

 that the formation of the contractile vacuoles depends 011 differences in the 

 tonicity of the protoplasm and the surrounding medium ; they also raise the 

 suspicion that many species of marine Protozoa may be only different forms., 

 due to change of medium, of fresh-water species, or vice versa. 

 For the excretory vacuole-system of Opalina, see p. 447. 



Insoluble excretion-masses are often formed in great quantity in 

 the bodies of Protozoa. Such substances take the form of crystals 

 or grains of various kinds, and often of pigment. An example of 

 such a substance is the melanin-pigment of the hsemamoebse (p. 359), 

 which appears to be a derivative of the haemoglobin of the infected 

 blood-corpuscle. Pigment may arise also by degeneration of 

 superfluous chromatin extruded from the nucleus, as in Actino- 

 sphcerium (p. 209), or by degeneration of nuclei, as in abnormal 

 oocysts of Cydospora caryolytica (p. 364). 



The cytoplasm of Paramecium contains crystals which have been studied 

 by Schewiakoff (206), who finds that they consist of calcium phosphate, either 

 Ca 3 (P04) 2 or Ca 2 Ho(P0 4 )2. When the Paramecia were starved, the crystals 

 disappeared completely in one or two days ; if then the Paramecia were 

 supplied with food again, the crystals reappeared. Schewiakoff was never 

 able to observe that the crystals were ejected from the anus but they were 

 seen to collect round the contractile vacuole. He is of opinion that the 

 insoluble phosphate is dissolved in the enchylema. or is converted into the 

 soluble form CaH 4 (P0 4 ) 2 , and then eliminated by the contractile vacuole. 



Insoluble excretion-masses may be simply extruded from the 

 body, a process which commonly takes place at certain crises, as, 

 for example, prior to encystment. Or, on the other hand, they 

 may remain in the protoplasm, and are finally abandoned in the 

 residual masses left over during reproductive phases, as seen com- 

 monly in the sporulation of various types for example, the 

 hsemamcebae already cited and other Sporozoa. In such cases the 

 young individuals are formed of protoplasm free from the coarse 

 excretion-granules, and the body of the parent, so much as is left 

 of it, dies off and disintegrates. In some cases, however, the young 

 individuals formed contain enclosures derived from the parent-body, 

 as, for example, the crystal-bearing swarm-spores of Radiolaria 

 (p. 254) ; but in such cases the enclosure is probably of the nature of 

 reserve-material. 



Secretion, more or less rapid, of various substances can be 

 observed without difficulty in various Protozoa. Examples are the 



