88 AN INTRODUCTION TO ZOOLOGY 



by means of its chlorophyll. Euglena will swim away from the 

 direct rays of the sun. Direct sunlight will kill the organism if 

 allowed to act for a long time. If a drop of water containing 

 Euglena is placed in the direct sunlight and then one half of it is 

 shaded, the animals will avoid the shady part and also the direct 

 sunlight, both of which are injurious to them, and will remain in a 

 small band between the two in the light best suited for them, 

 that is, their optimum (Fig. 41). By shading various portions of 

 the body of a Euglena it has been found that the region in front 

 of the eye spot is more sensitive than any other part. It should 

 be noted that when Euglena is swimming through the water it is 

 this anterior end which first reaches an injurious environment; 

 the animals give the avoiding reaction at once, and are thus carried 

 out of danger. 



3. PLASMODIUM 



(Plasmodium vivax Grassi) 



The Sporozoa are parasitic Protozoa and are responsible for 

 many of the most malignant animal diseases. One of the best 

 known of all Sporozoons is Plasmodium vivax, which causes Mala- 

 rial Fever. This minute animal was discovered in the blood of 

 malaria patients by a French military doctor, Laveran. It was 

 suggested by this investigator, in 1891, that the parasite is prob- 

 ably transmitted from man to man by some blood-sucking insects, 

 and this hypothesis was proved to be correct by the work of Major 

 Ross in 1899. Not only was it demonstrated that malaria is 

 spread by insects, but it was proved that human beings can only 

 become infected by the bite of a mosquito belonging to the genus 

 Anopheles. The two most common genera of mosquitoes are 

 Culex and Anopheles. One of the easiest methods of distinguish- 

 ing one from the other is by observing their position when at 

 rest. It will be found that the harmless Culex holds its abdomen 

 approximately parallel with the surface on which it alights, 

 whereas the abdomen of Anopheles is held at an angle. 



There are three well-known types of malaria; these may be 



