The Protozoa 



131 



each is a separate and distinct animal. Some of these cells are somatic 

 and nutritive, while others are germ-cells or reproductive cells. Here 

 we come in contact with the lowest form of sex life. Any organ which 

 produces sex cells is known as a gonad. There are certain germ cells 

 in a colony of Volvox called parthenogonadia ( ). 



These divide into many cells which drop into the center of the mother 



colony and finally escape through a 

 break in the wall. There are other 

 germ cells, however, which are also 

 produced ; the smaller are called sper- 

 matozoa or microgametes. These are 

 the male germ cells, while the larger 

 ones, called macrogametes or eggs, are 

 the female germ cells. The eggs are 

 fertilized by the spermatozoa, and, 

 after passing through a resting stage, 

 develop into new colonies. 



Colonies may be of one sex only. 

 In such cases the male colonies can be 

 recognized by the sperm pockets 

 arranged in a wide belt around the 

 middle of the colony with the poles 

 free from cells. 



There is a distinct difference be- 

 tween a colony of single-celled animals 

 of this kind and a tissue. A tissue is a 

 sheet of similar functioning cells, such sheet being combined with others 

 to form an organ, 'while the organs, taken together, form a complete 

 single individual. In Volvox there is no such grouping of sheets of 

 similar functioning cells, Each cell is complete and distinct in itself 

 and is as much an individual as Amoeba or Euglena just studied, except 

 that it is attached to its fellows. 



Fig. 49. Volvox. 



The individual cells are united by radi- 

 ating strands of protoplasm. A, a mature 

 colony; a, spermaries; g, ovaries. B, zy- 

 gote resulting from the fusion of the 

 gametes. C, two sperm. D, egg. (From 

 West, after Klein.) 



PLASMODIUM MALARIAE 



The malarial organism, Plasmodium malariae (Fig. 50), a member 

 of the class Sporozoa, nearly all of which are parasitic, lives in the 

 human body. Human blood contains minute circular disks, known as 

 red blood corpuscles, within which the malarial organisms may be found 

 in persons who are suffering from malaria, or chills and fever. The 

 organism first appears as an extremely minute body, in shape somewhat 

 like the Amoeba, though much smaller. It increases in size. After 

 reaching a size which nearly fills up the red blood corpuscles, it breaks 

 up into twelve to sixteen small spores. The blood corpuscles now break 

 into pieces and the spores are liberated into the liquid blood. Each 



