142 GENERAL ZOOLOGY 



about a day and a half, then divides into several little 

 spores (Fig. 63). The infected corpuscle finally bursts open, 

 setting the spores and a poison free in the blood. The 

 poison causes the febrile condition which a patient shows. 

 The little spores enter new corpuscles, grow for two days, 

 and then release another crop of spores. There is thus a 

 periodic recurrence of fever, every third day, which is corre- 

 lated with a certain stage in the development of the parasite. 

 ; Three things are necessary before malaria can become 

 established in a region: (1) infected people, (2) anophelene 

 mosquitoes, and (3) warm climate. A man may carry the 

 parasites and serve as a " reservoir" long after he has 

 ceased to have fever. The mosquito cannot transmit the 

 disease as soon as it draws blood from an infected person, 

 but must " incubate" the parasite for about twelve days. 

 During this incubation period the germ cells of the parasite 

 conjugate and young spores are produced which take up 

 their position in the mosquito's salivary glands, ready to 

 enter the next man who is bitten. 



CLASS IV. INFUSORIA 



In this class of protozoans cilia are the characteristic 

 organs of locomotion (Fig. 62, H-K). Cilia are much like 

 flagella, but are smaller and always occur together in large 

 numbers. There is always a cell-membrane or pellicle 

 covering the body of a ciliate and two nuclei are usually 

 present. The larger, or trophonucleus, is concerned with 

 ordinary metabolic activities; the smaller micronucleus is 

 usually active only during fission or conjugation. The cells 

 of some ciliates are the most complicated in the animal 

 kingdom. It will be well, therefore, to consider the activi- 

 ties of a typical representative in some detail in order that it 

 may be compared with the amoeba (page 135), which is one 

 of the simplest of "cells. For this purpose the "bell animal- 

 cule" Vorticella, is chosen. 



