158 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



Euglena. These living cells may often be seen with the naked eye as 

 whitish specks, moving slowly near the surface of a laboratory hay 

 infusion that has been standing for some time. There are several 

 different species commonly found, some larger than others, although 



in a drop of infusion 

 much variation in size 

 within the same species 

 may be found. The 

 class, Infusoria, con- 

 tains a large number of 

 forms, one of which, 

 longitudinal fiber Paramecium, or the 

 — -trichocvst "slipper animalcule," is 



.ei\ 



IOC 



^basal i^TamAs. 



1-^- - thread of attachment 



Diagram showing structure of the pelHcle in Para- 

 nicciurri nnillimicronucleata. Under high power 

 of the microscope the peUicle is seen to form minute 

 hexagonal areas, from the center of each of which a 

 cilium protrudes. The cilia arise from basal gran- 

 ules (microsomes) which are located on strands of 

 protoplasm (longitudinal fibers). Where do the 

 t richocysts lie with reference to the cilia .•* Where 

 are the openings through which the trichocysts are 

 discharged? (After Lund.) 



very common. It has a 

 somewhat flat, elliptical 

 body with the anterior 

 thinner end more blunt 

 and the broader poste- 

 rior end more pointed. 

 The cell body of Para- 

 mecium is almost trans- 

 parent and is made up 

 of an outer, non-granu- 

 lar layer, the ecto'plasm, 

 and an inner semifluid, granular layer, the endoplasm. The ectoplasm 

 is covered with a delicate, elastic, but lifeless covering called the 

 pellicle. Under it is the living cell membrane and through the pellicle 

 project numerous threads of protoplasm, the cilia, which are distrib- 

 uted over the surface of the body in regular rows. The cilia are 

 quite uniform in size except at the posterior end of the cell, where 

 they are a little longer. It is by means of a lashing movement of 

 these cilia that locomotion takes place. Embedded in the clear ecto- 

 plasm are also fotmd nimierous defensive structures, called trichocysts. 

 Under certain conditions, delicate filaments or threads are discharged 

 from them which serve as organs of offense and defense. It is be- 

 lieved that they may contain minute quantities of poison which 

 paralyzes other protozoa. 



On one side a depression, the oral groove, runs diagonally from the 

 anterior end of the body to about the middle. This oral groove ends 

 in a gullet, which in turn leads to the interior of the cell. The 



