THE PROTOZOA 



1-27 



(3) Thermotropism, meaning a reaction to heat; 



(4) Phototropism, meaning a reaction to light; 



(5) Electrotropism, meaning a reaction to an electric current; 



(6) Geotropism, a reaction to gravity; 



(7) Chromotropism, a reaction to color; 



(8) Rheotropism, a reaction to current. 



Amoebae move away from strong light, so that they are said to be 

 negatively phototropic. They are also negatively thigmotropic. 



If an action is self-imposed it is said to be autogenous ( ) ; 



if an external object causes a reaction, whether such object be located 

 within or without the body, the action is known as etiogenous ( ). 



EUGLENA 



This little organism (Fig. 48) moves about like a full-fledged ani- 

 mal although it has chlorophyl in its body and manufactures its food 



as does the plant; and it does this notwith- 

 standing the fact that it has a mouth and 

 gullet. 



Euglena belongs to the Class Mas- 

 tigophora ( ), which 



means that there is a whip-like flagellum 

 protruding from its anterior end. Several 

 animals must be grouped together in order 

 that the naked eye may see any organisms 

 present. When there are many about there 

 is a characteristic green color given the sur- 

 rounding water. 



EXTERNAL AND INTERNAL 

 FEATURES 



: Euglena viridis is a single celled 

 elongated animal pointed at the posterior, 

 and blunt at the anterior end. As in 

 the stigma; r, reservoir; c.v., con- Amoeba, there are two layers in the cyto- 



tractile vacuoles ; chr, chromato- , , , 



phors; am, pyrenoids with sheaths plasm, the ectosarc, a dense outer layer, and 



the endosarc, a more fluid central mass. 

 There is a thin cuticle running in parallel 

 E a free thickenings around the bodv of the animal 



undergoing 1 



longitudinal division. F and G, divi- in an oblique direction so that it appears 



sion of an encysted form. (A-D, 



after Bourne; E-G, after Stein.) Striated. 



The mouth is a funnel-shaped depression lying a little to one side 

 of the center of the anterior blunt end. The gullet is a continuation of 

 this depression. It looks like a duct, and connects with a large spher- 

 ical vesicle, the reservoir, into which several minute contractile vacuoles 

 discharge their contents. 



Fig. 48. 



A. Euglena viridis; m, the so- 

 called mouth ; n, the nucleus ; e, 



