SURVEY OF UNICELLULAR ANIMALS 53 



of energy has in the Sporozoa, as elsewhere in the Animal Kingdom, 

 resulted in a degeneration of structures necessary for a free life, 

 and an elaboration of the reproductive processes to ensure that 

 the parasite secures access to the proper host. For, in general, 

 each Sporozoon is adapted to live in one (or two) particular 

 species of animal — indeed probably every species of higher 

 animal has at least one Sporozoon specially fitted to live at its 

 expense. 



Monocystis is a common Sporozoon that spends its entire life 

 in the body of an Earthworm. The 'adult' Monocystis is an 

 elongated cell living in a part of the reproductive system (seminal 

 vesicles) and securing its nourishment from the developing germ 

 cells of the worm. Here food is plentiful, and accordingly much is 

 stored for use during the complex reproductive changes which 

 terminate in the production of resistant spores. These eventually 

 are discharged from the body of the worm, and trust to chance 

 that entrance may be gained to the body of another worm in order 

 that the life cycle may be repeated. (Fig. 25.) 



Malarial fevers would seem to be of more importance than dis- 

 eases of the Earthworm, though our knowledge of the Sporozoa 

 that are responsible for these human maladies has been made 

 possible by basic studies on the Protozoa as a whole. Malaria is 

 transmitted to Man solely by diseased female Mosquitoes that 

 inoculate into the blood stream a Sporozoon of the genus Plas- 

 modium. Once in the human blood, the parasite enters a red blood 

 cell and begins its complicated life history. Multiplication of the 

 parasite until thousands are present results in the periodic libera- 

 tion of poisonous material in the blood which produces the alter- 

 nate chills and fever. In order to complete its life history the 

 parasite must again be taken into the body of a Mosquito by the 

 latter biting an infected person. (Figs. 223, 246, 247.) 



D. Infusoria 



The ciliated Protozoa, constituting the class Infusoria, probably 

 represent the most complex development of the unicellular plan 

 *of animal structure. Infusoria have afforded ready material for 

 the study of various physiological problems, not only because some 

 of the species are relatively large, but also because, in general, 

 they lend themselves most readily to experiment. Most of the 

 Infusoria are free-living in fresh and salt water, though not a few 



