264 PARAMOECIUM CHAP. 



B, the same in optical section showing cuticle (CM), cortex (corf), and medulla 

 (med] ; buccal groove (buc. 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 permanence of contour is due to the presence, as 

 in Euglena, 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 endoplasm simply by 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, 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. 



