CORTEX AND MEDULLA 107 



(mlh] and gullet (gul) ; several food vacuoles (/. vac], and the two 

 contractile vacuoles (c. vac). 



B, the same in optical section, showing cuticle (cti), cortex (cort), and 

 medulla (med) ; buccal groove (buc. gr), mouth, and gullet (gul) ; 

 numerous food vacuoles (f. vac) circulating in the direction indicated 

 by the arrows, and containing particles of indigo, which are finally 

 ejected at an anal spot ; nucleus (nu) and paranucleus (pa. mi), and 

 trichocysts, some of which (trch) 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 tricho- 

 cyst-threads, which project considerably beyond the cilia. 



D, diagram of binary fission : the paranucleus (fa. mi) has already 

 divided, the nucleus (nu) is in the act of dividing. 



(D after Lankester.) 



rendered somewhat concave by the twist a wide shallow 

 groove beginning at the broad end and gradually narrowing 

 to about the middle of the body, where it ends in a tolerably 

 deep depression. 



The groove is called the buccal groove (Fig. 21, A & B, 

 buc. gr.} : at the narrow end is a small aperture the mouth 

 (mth\ which, like the mouth of Euglena (Fig. 5) leads into 

 the soft internal protoplasm of the body. The surface of 

 the creature on which the groove is placed is distinguished 

 as the, ventral surface, the opposite surface being upper or 

 dorsal : the broad end is anterior, the narrow end posterior, 

 the former being directed forwards as the animalcule swims. 

 These descriptive terms being decided upon, it will be seen 

 from Fig. 21 A, that the buccal groove begins on the left side 

 of the body, and gradually curves over to the middle of the 

 ventral surface. 



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 permanence of 

 contour is due to the presence of a tolerably firm though 

 delicate cuticle (cii) which invests the whole surface. 



