2(14 PARAMCECIUM chap. 



B, the same in optical sections showing cuticle (c?0> cortex (c<?W), and medulla (^wi-rf); 

 buccal groove (biic. gr), mouth, and gullet (^t//) ; numerous food vacuoles 

 (y. vac) circulating in the direction indicated by the arrows, and containing 

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

 micronucleus (/«. nu), and trichocysts, some of which {tj-ch) are shown with 

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



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

 which project considerably beyond the cilia. 



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

 meganucleus («w) IS in the act of dividing. (From Parker's .5/(7/(j^_j' ; D, after 

 Lankester.) 



As the animal 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, en) which invests the whole surface. 



'I'he protoplasm thus enclosed by the cuticle is distinctly 

 divisible into two portions — an external somewhat dense 

 la)'er, 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 Amceba 

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

 plasm simpl)' bv the absence of granules. In Paramoecium 

 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 (/ vac, see p. 266) move 

 freely in it, whereas they ne\er 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 mvoplia/i layer. 



