LOCOMOTION 



49 



in the oxidation process, are collected by tiny radiating canals 

 and emptied into a central contractile vacuole at each end of the 

 organism. When these vacuoles have attained a certain size, the 

 cytoplasm surrounding them contracts and discharges the wastes 

 through the cell membrane. The contractile vacuole is constantly 

 filling and discharging. 



Locomotion. Cytoplasmic structures called cilia project through 

 the cell membrane, and, by rapidly lashing back and forth, propel 

 the animal through the water. The cilia are ar- 

 ranged in rows all over the body. All the cilia in 

 a row beat at the same time. If, with a needle, 

 the thread of cytoplasm which controls the beating 

 of a row of cilia be cut, the entire row will cease to 

 beat and remains paralyzed. Due to the more 

 rapid beating of the cilia in the groove, the animal 

 rotates and proceeds in a spiral rather than a 

 straight path. It usually progresses with the blunt 

 or anterior end forward, but can reverse its cilia 

 and travel equally well with the pointed end for- 

 ward. Such a reversion often takes place when it 

 meets an obstacle. If confined in close quarters, 

 the Paramecium can pass through small openings 

 due to the elasticity of its body. 



Reproduction. Reproduction occurs by fission, 

 as in amoeba. Each of the two nuclei divides and 

 each daui^hter cell contains a macronucleus and 

 micronucleus. A new groove and g.ullet are formed 

 in one of the cells and two new contractile vacu- 

 oles appear. When cell division or binary fission goes on indefi- 

 nitely, the cells sometimes lose their vitality. When this happens 

 they become smaller, and, in some cases, distorted. Under such 

 conditions, two Paramecia come together. The cell membranes 

 are dissolved at the point of contact and a bridge of cytoplasm 



A Paramecium 

 reaches a maximum 

 size. The two nu- 

 clei elongate, a con- 

 striction appears, 

 and the two daugh- 

 ter cells split apart. 

 This is reproduc- 

 tion by fission. 



