90 THE PROTOZOA 



appears to be mainly carbon dioxide, and apparently serves an hydro- 

 static purpose, allowing the heavy forms like Difflugia to raise or 

 lower themselves in the water. 



E. ENCYSTMENT 



Encystment is undoubtedly a widespread phenomenon among the 

 Sarcodina, although it is apparently absent altogether in the marine 

 forms (Radiolaria and Reticulariida). With the exception of some 

 Heliozoa, it has not been extensively studied, and the few observa- 

 tions are frequently contradictory. There is a general agreement, 

 however, that its object is to protect the individual during periods of 

 drought, cold, or during periods of reproduction. Thus a heliozob'n, 

 when its environment becomes unsuitable, draws in its pseudopodia, 

 loses its ectoplasmic vacuoles, and secretes a double-layered coating, 

 the inner layer being gelatinous at first, but later like a membrane. 

 The outer layer is warty, and composed usually of silicious plates, 

 cemented together by a silicious jelly. If multinucleate, most of the 

 nuclei are absorbed, about 5 per cent remaining intact (Hertwig, for 

 Actinosphczrium). When conditions are again suitable, the animal 

 absorbs water, swells, becomes vacuolated, bursts its membrane and 

 outer cyst, and as a free-swimming heliozoon develops pseudopodia, 

 and again leads an active life. Brauer ('94) and Hertwig ('98) have 

 shown that in Actinosphcerium encystment is accompanied by numer- 

 ous phenomena of plastogamy and karyogamy^ In some forms of 

 Heliozoa, as in Vampyrella, the outer cyst is composed of other mate- 

 rial than silica, usually of cellulose, while in the fresh-water Rhi- 

 zopoda the cyst coatings are chitinous. Here also the conditions are 

 often changed by the presence of a shell, the cyst membrane in such 

 cases covering over only the mouth-opening, although in one form at 

 least {Euglypha) a second cyst membrane envelops the whole animal 

 inside of the shell (see Fig. 17, p. 47). 



F. NUTRITION 



The food of Sarcodina consists of vegetable substances, of flagellates, 

 Infusoria, and not infrequently of larger animals, such as rotifers or 

 small Crustacea. In the simpler forms there is no region set aside for 

 the ingestion of food particles, but any portion of the body-surface can 

 function as a mouth. When food particles strike the body, the stimu- 

 lus causes the protrusion of specialized pseudopodia, which gradually 

 surround the object. Zacharias ('93), upon rather uncertain data,. 



i Cf. pp. 236 and 237. 



