2l6 FRESH-WATER BIOLOGY 



Waste fluids resulting from the metabolism of the cell are col- 

 lected in contractile vacuoles in most of the fresh-water Sarcodina 

 and thereby removed. Surplus water which has been ingested is 

 eliminated at the same time and possibly carbon dioxid may also 

 be extracted by the contractile vacuole. In those forms, however, 

 which do not possess contractile vacuoles, the waste fluids and gases 

 escape from the general surface of the body. 



Respiration in the Sarcodina is performed by an interchange of 

 gases through the body wall. Oxygen is received from the sur- 

 rounding water and carbon dioxid transmitted to it by osmosis. 

 The symbiotic relationship between algae and some forms of Sar- 

 codina, without doubt, has an important respiratory as well as 

 nutritive function, the plants furnishing oxygen and carbohydrates 

 to the animals, while the latter supply carbon dioxid and nitroge- 

 nous waste for the algae. 



Reproduction in the Sarcodina takes place either by the division 

 of the body into two parts, a process commonly called fission; by 

 the separation from the parent of one or more small masses of 

 protoplasm known as buds; or by the production of swarm spores. 



In fission, or simple cell division, which is common among fresh- 

 water forms, the nucleus divides first and this is followed by the 

 separation of the cytoplasm into two parts, each of which encloses 

 a portion of the original nucleus. Growth proceeds until maturity 

 is reached, when the process of division is repeated. 



When an envelop is present the enclosed body may divide by 

 fission after which one portion creeps out at the aperture and con- 

 structs a new shell about itself. If the envelop be chitin and with- 

 out distinct elements it is gradually developed at the periphery 

 of the newly separated individual, but if it be of regular chitinous 

 or siliceous plates, these elements, in some cases at least, are de- 

 veloped in the cytoplasm of the parent and pushed out to form the 

 new envelop of the separating bud. 



That the nucleus is concerned with cell division has long been 

 known. Recent observations, however, have thrown light upon 

 the presence of extranuclear material scattered throughout the 

 cytoplasm in many Sarcodina as well as other protozoa. This 

 material has the form of minute granules termed " idiochrornidia " 



