PARAMCECIUM 



117 



unfavourable medium it " reverses " for a short distance, 

 turns on its side, and goes forward at an angle to the 

 original path. Among the ciUa there are small cavities in 

 the cortex, in which lie fine protrusible threads (" tricho- 

 cysts "). These, though parts of a cell, suggest the thread 

 cells of Coelentera, but are probably of the nature of 

 mooring threads effecting attachment to solid objects. 



AAA 



Fig. 50. 



-Paramcecium in longitudinal optical section, 

 and dividing. — After Butschli. 



C.V., Contractile vacuole ; MY., longitudinal " myophan " striations ; 

 MA., macronucleus ; MI., one of two micronuclei ; F.V., food 

 vacuole ; CA., a canal in the cytoplasm entering the contractile 

 vacuole which is bursthig through the cortex ; TR., trichocysts at 

 the roots of the cilia (CI.) ; MO., " mouth " leading into gullet. In 

 the right-hand figure D.L. is the transverse dividing line ; the dumb- 

 bell-like elongations of the macronucleus (MA.) and micronuclei 

 (MI.) ; P. A., a "potential anus or weak spot," where debris may 

 be got rid of. 



The cortical layer is contractile, an^i is distinctly fibrillated. 

 In the substance of the cell lie two nuclei, the smaller 

 " micronucleus " lying by the side of the larger " macro- 

 nucleus." Food vacuoles occur as in the Amoeba, and the 

 digestive process appears to be similar ; but Paramcecium 

 is remarkable for the strength of the acid which it secretes 

 into the vacuoles. There are two contractile vacuoles, 

 from which fine canals radiate into the surrounding proto- 



