264 PARAMCECIUM CHAP. 



B, the same in optical sections showing cuticle (cu), cortex (corf), and medulla (med)\ 

 buccal groove (due. gr), mouth, and gullet (gul} ', numerous food vacuoles 

 (_/". vac) circulating in the direction indicated by the arrows, and containing 

 particles of indigo, which are finally ejected at an anal spot ; meganucleus (nu), 

 micronucleus (pa. nu), and trichocysts, some of which (trek) are shown with 

 their threads ejected. The scale to the right of this figure applies to A and B. 



C, a specimen killed with osmic acid, showing the ejection of trichocyst-threads, 

 which project considerably beyond the cilia. 



D, diagram of binary fission ; the micronucleus (pa. nu), has already divided, the 

 meganucleus (nu) is in the act of dividing. (From Parker's Biology: D, after 

 Lankester.) 



As the creature swims its form is seen to be permanent, 

 exhibiting no contractions of either an amoeboid or a 

 euglenoid nature. It is however distinctly flexible, often 

 being bent in one or other direction when passing between 

 obstacles, such as entangled masses of weed. This perma- 

 nence of contour is due to the presence of a tolerably firm 

 though delicate cuticle (B, cu) which invests the whole surface. 



The protoplasm thus enclosed by the cuticle is distinctly 

 divisible into two portions an external somewhat dense 

 layer, the cortical layer or cortex (cort\ and an internal more 

 fluid material, the medullary substance or medulla (med). It 

 will be remembered that a somewhat similar distinction of 

 the protoplasm into two layers is exhibited by Amoeba 

 (p. 231), the ectoplasm being distinguished from the endo- 

 plasm simply by the absence of granules. In Paramcecium 

 the distinction is a far more fundamental one : the cortex is 

 radially striated and is comparatively firm and dense, while 

 the medulla is granular and semi-fluid, as may be seen from 

 the fact that food-particles (f. vac, and see p. 266) move 

 freely in it, whereas they never pass into the cortex. The 

 medulla has a reticular structure similar to that of the 

 protoplasm of the ordinary animal-cell, consisting of a 

 delicate granular network the meshes of which are filled 

 with a transparent material. In the cortex the meshes of 

 the network are closer, and so form a comparatively dense 

 substance. The cortex also exhibits a superficial oblique 

 striation, forming what is called the myophan layer. 



