212 FRESH-WATER BIOLOGY 



most of the Sarcodina there is a marked differentiation between 

 the endoplasm and ectoplasm, the difference consisting in the 

 greater density and opaqueness of the inner region. This dis- 

 tinction between endoplasm and ectoplasm reaches a high degree in 

 certain marine forms, the Radiolaria, where a distinct perforated 

 membrane, the " central capsule," separates the two regions. None 

 of the fresh- water forms possess a " central capsule." The greater 

 density of the endoplasm is accounted for by the character of the 

 inclusions suspended in it and by the size of the vesicles which 

 make up its structure. 



The inclusions consist of various elements: food which is to 

 be built into body protoplasm, products which may enter into the 

 composition of the shell, waste material on the way to the exterior, 

 or foreign elements which have no part in the physiology of the 

 animal. In some Sarcodina algae or bacteria are constant features 

 of the endoplasm, this symbiotic relationship being apparently 

 essential to the life of the protozoon. 



The nucleus is confined to the endoplasm. In a few Sarcodina 

 condensed nuclear elements have not been observed, but in these 

 cases the chromatin is without doubt diffused throughout the cell 

 and has the same functional value as a centralized body. 



Usually a single nucleus is present, often two is the normal num- 1 

 ber, but in some forms several hundred or even several thousand 

 have been counted in certain stages. 



Commonly the nucleus is spherical, but may be modified in 

 form, due to the shape of the body and to the flexibility of the 

 nuclear membrane which sometimes permits considerable deformity. 



In most Sarcodina the chromatin within the nuclear membrane 

 is arranged in one or more masses, but in some of the Heliozoa it 

 is arranged in a network not unlike that of the cell of a metazoon. 



Contractile vacuoles, the function of which is the extraction of 

 waste fluids and gases, are not found in all Sarcodina. These are 

 absent in many of the marine forms and in some fresh- water genera. 

 When present, the number varies from one to many. Habitually 

 the contractile vacuole is spherical but in some species it assumes a 

 characteristic lobed form. The position of the contractile vacuole 

 is not always fixed but may frequently be shifted about by the 



