146 ANIMAL BIOLOGY 



like flagellates lead up to the simpler algae and thence on 

 to the higher plants; the animal-like flagellates lead on 

 to other groups of animals. There is evidence that the 

 flagellates are related to the simplest of all known organ- 

 isms, the bacteria, which we may regard as standing at 

 the very root of the tree of life. 



Some of the flagellates that live within the bodies of 

 animals are the causes of very severe diseases. Chief 

 among these forms are the trypanosomes which are found 

 generally in the blood of the infected animal. Nagana, 

 which carries off thousands of horses in Africa, is caused by 



a species of trypanosome. The 

 disease is conveyed by means 

 of the bite of the tsetse-fly, 

 much as malaria is carried from 

 one person to another by the 

 mosquito. One of the worst 

 scourges of humanity that is 

 known, the " sleeping sickness " 



FIG. 119. Trypanosomes. r A r i i , i, 



of Africa, which is estimated to 



have carried off in the district of Uganda some one hun- 

 dred thousand natives in four years, is caused by another 

 trypanosome, Trypanosoma gambiense. The disease in its 

 later stages is accompanied by extreme drowsiness which 

 gave it its name, and it almost always results in the death 

 of the patient. It is now known that this disease is con- 

 veyed from man to man by a species of tsetse-fly, but one 

 that is different from the species that carries nagana. 

 In the group Sarcodina the body does not have a fixed 

 outline, for it has the power of pushing out and with- 

 drawing projections called pseudopodia (false feet) which 

 serve both for locomotion and the capture of food. One 

 of the simplest and best known of the group is the common 

 Amoeba proteus. The organism appears like a mass of 



