172 INVERTEBRATE ZOOLOGY 



INFUSORIA 



A FLAGELLATE INFUSORIAN. EUGLENA 



This single-celled organism, which combines the characters of 

 animals and plants, is often so plentiful in pools and ditches 

 that it makes the water green. It is a minute elongated inf uso- 

 rian, one end of which is pointed and the other blunt; in 

 the latter end is a deep depression, from the bottom of which 

 springs a long, thread-like, vibratile flagellum. The body is cov- 

 ered by a very delicate cuticula ; an oral groove and a mouth are 

 not present. The animal is colored green by the presence of 

 chlorophyll in its body. 



Mount a drop of water containing Euglena on a slide and 

 study it under the microscope. Observe its shape and color ; 

 also its swimming motions and the motions of the flagellum. 

 The latter organ will be seen to be at the anterior end of the 

 body ; it is always in advance as the animal swims. In some 

 flagellates the flagellum is at the posterior end. Whether the 

 flagellum in any species is at the anterior or the posterior end 

 of the body depends upon the direction the vibratile motion of 

 the flagellum takes. If the motion begins at the base of the 

 flagellum and proceeds towards its tip, the animal's body will be 

 driven ahead with the flagellum at the rear, while if the motion 

 begins at the tip of the flagellum, the body will be drawn after 

 it. Note the extreme plasticity of the body. It can assume 

 a variety of shapes, and will often be seen swimming by the 

 alternate contraction and expansion of the body, like a worm. 



Exercise l. Draw a number of simple outlines of the body 

 showing its shape at different times. 



