52 



ANIMAL BIOLOGY 



individuals occur that suggest a possible method of origin of the 

 multicellular body of higher animals. (Figs. 29, 30.) 



C. Sporozoa 



Although parasitic species are found in all four classes of Pro- 

 tozoa, the Sporozoa have the distinction of living solely at the 



Fig. 25. — Life cycle of a Sporozoon, Monocystis. A. spore consisting of a 

 spore case enclosing eight sporozoites; B, transverse section of same: C and D, 

 liberated sporozoites; E, sporozoite after entering 'sperm-sphere' of Earth- 

 worm; F and G, growth until fully developed trophozoite is formed surrounded 

 by the degenerate remains of sperm-sphere with flagella of sperm; H, two 

 trophozoites that have become free from degenerate sperm-sphere and united 

 as gametocytes; I, encystment of gametocytes; J, division of nuclei and cyto- 

 plasm to form gametes; K, union of the gametes to form zygotes, residual 

 cytoplasm of gametocytes in center of cyst; L, cyst containing many sporo- 

 zoites formed by secretion of a spindle-shaped spore case around each zygote 

 which then divides to form eight sporozoites. These become arranged as in A 

 and are ready to be transferred to another host. (From Curtis and Guthrie.) 



expense of other organisms, their hosts, in which they create dis- 

 turbances of more or less severity which frequently result in dis- 

 ease. The successful attempt to get a living with little expenditure 



