152 '■"f ANIMAL KINGDOM 



feeding. Many marine protozoa and most parasitic species do not ingest 

 food, hence they do not tend to accumulate water, and have little use 

 for a contractile vacuole. 1 he exact mechanism by which the contractde 

 vacuole fills with water is unknown, but it is emptied by the contraction 

 of the surrounding cytoplasm which shifts from a sol to a gel as it con- 

 tracts and forces the bubble to burst to the outside. 



48. Class Flagellata 



Flagellates are spherical to elongate protozoa with a simple, cen- 

 trally located nucleus and from one to several flagella at one end. The 

 group is large, including half the known living protozoan species. Most 

 of these are small and difficult to study but a few, such as members of 

 the genus Euglena (Fig. 8.4), are large and easily obtained. A study of 

 Eugleua will introduce the class. 



Euglena. Euglenas (£. vindis and E. gracilis are common species) 

 are elongate flagellates 50 to 100 or more micra long. At the anterior 

 end a deep depression forms the gullet. Although euglenas have never 

 been observed to feed, members of the genus Peranema of the same 

 family use the gullet for swallowing prey. The body is covered with a 

 delicate pellicle showing spiral thickenings. Beneath the pellicle, in- 

 visible without special stains, is a layer of contractile fibrils with which 

 the organism can change shape. Euglenas often creep upon the bottom 

 in a wormlike fashion. A single contractile vacuole lies next to the 

 gullet and empties into its base. The large nucleus is in the posterior 

 third of the body. 



Scattered in the cytoplasm are chloroplasts and paramylum bodies. 

 Chloroplasts are bright green with their contained chlorophyll; in the 

 light they are able to carry on photosynthesis, like the chloroplasts of 

 plants. The arrangement of chloroplasts is used in the identification 

 of species. In E. viridis they are large and form a rosette (Fig. 8.4). In 

 E. gyacilis and in several other common species they are small and 

 numerous, obscuring all other internal structures except the red pigment 

 spot. The transparent, colorless paramylum bodies are a form of poly- 

 saccharide unique to the euglenas, difl:erent from both the glycogen of 

 other animals and the starch of plants. The arrangement of these also 

 varies among the species. They are formed during photosynthesis, and 

 if they are so numeious as to obscure other structures their numbers 

 can be reduced by keeping the euglenas in the dark a day or two. A 

 single long flagellum which protrudes from the gullet is used for swim- 

 ming. It is formed by the fusion of two flagella that arise from two basal 

 granules in the base of the gullet. At the point of fusion (Fig. 8.4) is a 

 transparent swelling, the photoreceptor. Next to this in the wall of the 

 gullet is a red pigment spot. 



Swimming is a complex movement. The sideways lashing of a single 

 flagellum is like one-armed swimming; the body is thrown forward bui 

 also to one side at each stroke. In Euglena the flagellum usually bends 

 toward the side bearing the pigment spot, and if this stroke were merely 

 repeated over and over the organism would move in a circle with the 



