106 



THE SMALLEST LIVING THINGS 



One great group of Sporozoa, Gregarinida, are parasites of 

 digestive tracts and body cavities of invertebrate animals; an- 

 other great group Coccidia (Fig. 59), are parasitic in the 

 epithelial cells of both vertebrates and invertebrates; others are 

 parasites of the blood stream, e.g., Haemosporidia (Fig. 60), 



Fig. 60— A PARASITE, HAEMOGRE- 



GARINA STEPANOWI, IN A 



BLOOD CELL OF A TURTLE 



This organism always curls up in this character- 

 istic way alongside the nucleus 



From a photomicrograph by the author 

 Magnification, 700 



and still others of tissues, e.g., Neosporida. Gregarines are ap- 

 parently harmless to their hosts, but the other Sporozoa, with 

 their cell-destroying habits, are dangerous, frequently causing 

 disastrous epidemics both among men and among domesticated 

 animals, and are dreaded enemies of the breeders of fishes, silk- 

 worms, and bees. 



The life history of a typical sporozoan is definite, and with 

 some variations may be applied to all forms. I will illustrate 

 with Eimeria schubergi (Fig. 61), an intestinal parasite of the 

 common centipede and belonging to the same group as the form 

 which is fatal to all ground-moles (see page 104). The young 

 germ, known as a "sporozoite," is freed from its capsule by the 

 solvent action of the digestive fluids of the centipede. It pene- 

 trates an epithelial cell and grows into a large intracellular para- 

 site, the nucleus of the infected cell being pressed to one side 

 where it atrophies. The nucleus of the parasite meanwhile di- 

 vides several times, after which its cell body breaks up into as 

 many small cells as there are nuclei. These small cells, known 

 as agametes (Fig. 62), or asexual spores, break out of the 

 epithelial cell and penetrate other normal epithelial cells in which 

 the process of growth and sporulation is repeated. This is 



