GENERAL ZOOLOGY 



Cilium 



Pore of 

 trichocyst 



^Neuromotor" 

 fibril 



Basal granule 



Body of 

 trichocyst 



Fig. 8.19. Paramecium: rela- 

 tionship of pellicular structure 

 to ectoplasmic organelles. 

 (Adapted from E. E. Lund, 

 1935, University of California 

 Publications in Zoology, 

 vol. 39, printed by permis- 

 sion of the University of 

 California Press.) 



The greater part of the cell is composed of the endoplasm, which is suf- 

 ficiently fluid to allow circulation of food vacuoles and other inclusions. The 

 definite shape of the paramecium depends, therefore, on the relative rigidity 

 of the ectoplasm and the pellicle. Within the endoplasm lies the macronucleus, 

 related to the metabolic or vegetative activities of the cell, and the micro- 

 nucleus, concerned with heredity and reproduction. Paramecium aurelia has 

 two micronuclei, whereas P. mullimicronucleatum has many. The endoplasm 

 also contains two contractile vacuoles, one anterior and one posterior, and the 

 food vacuoles. Larger masses of various sorts may also be found, in addition 

 to the very small inclusions of the cytoplasm. 



Movement and Responsiveness. Locomotion in Paramecium is effected by 

 the action of the cilia, which by coordinated beating propel the animal in a 

 spiral course. An understanding of this process involves two problems: first, 

 that of explaining the operation of individual cilia; and second, that of 

 accounting for the integration of the activities of the individual cilia in such 

 a way as to provide for directed locomotion. The mechanism of ciliary action 

 is not well understood, but it is clear that the physical state or configuration 

 of the cilium must be altered between its "effective" stroke and its "recovery" 

 stroke. It can be observed that the cilium is relatively stiff^, and moves 

 rapidly, during the effective or driving stroke, and that it becomes relatively 

 limp and moves more slowly as it returns to its original position during the 

 recovery phase (Fig. 8.20). The factors governing these changes presumably 

 result from the interaction of the basal granule, or kinetosome, of the cilium 

 and the axial filament, which springs from the kinetosome and runs the length 

 of the cilium. Experiments have shown that a single cilium exhibits spon- 

 taneous movements as long as its connection with the kinetosome remains 

 intact. Without the kinetosome, the cilium is incapable of beating. This 

 indicates that a capacity for initiating activity resides in the kinetosome. It 

 seems reasonable to speculate that a common physicochemical mechanism 



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