284 GENERAL ZOOLOGY 



substances which it does not like, Euglena ceases moving, 

 contracts into a ball, and encysts itself, just as it does in 

 nature when it is surrounded by unfavorable conditions. 



The Malarial Parasite. Many one-cell animals are parasitic. 

 One of the most studied in recent years is the malarial para- 

 site of man (Plasmo'dium mala' rice, Fig. 142). The life-history 

 of this organism is long and complicated, but we can get a fair 

 understanding of its principal phases without discussing the 

 minutest details. 



In its simplest form the animal resembles an extremely 

 small amoeba. In that stage it is found in the red corpuscles 

 of human blood. There the parasite increases in size until it 

 almost fills the corpuscle (Fig. 142, 4) ; then it divides into 

 small bodies called spores (Fig. 142, 5a v ). The process of 

 spore-formation causes the chill that accompanies malaria. 

 When the spores burst from their spore-case (Fig. 142, 5a VI ) 

 and from the blood-corpuscles, a quantity of poisonous mate- 

 rial (represented by black dots in Fig. 142, 5 YI ) is released 

 and mingles with the liquid of the blood. This poisonous 

 material induces the fever which always follows the chill. 

 The released spores may enter red corpuscles again, and in 

 forty-eight hours in one type of malaria, and seventy-two 

 hours in another, form spores once more. 



Some of the amoebulce (amoeba-like stages) of the parasite 

 have a different history. While most of them in the red cor- 

 puscles of a person go on reproducing non-sexually, as just 

 described, some develop into a form which, by comparison 

 with higher animals, we call the female cell (Fig. 142, 5c n ), 

 and others into male cells (Fig. 142, 5Z> n *). If now the person 

 be exposed to the bite of a mosquito of the genus Anopheles 

 (see p. 63), the male and female cells of Plasmodium each 

 reach their full development in the human red blood-corpus- 

 cles, as these rest in the stomach of the mosquito. Leaving 

 the corpuscles, the two cells unite to form a worm-like cell 



