BIOLOGY AND HUMAN WELFARE 395 



gradual loss of the mental faculties and death. The discovery of 

 the cause has made possible intensive studies of therapeutic meas- 

 ures to combat it, and the test of upward of a thousand chemical 

 substances has resulted in the discovery of certain organic arsenic 

 compounds of considerable specific value if employed in the early 

 stages. This knowledge, supported by the enlightened attitude 

 that is gradually being taken toward the disease, offers a brighter 

 outlook for the future. While syphilis, of course, is not inherited, 

 much of it in the world today is due to the infection of infants 

 before or at the time of birth. (Fig. 249.) 



Fig. 249. — Treponema pallidum (the spiral bodies) in liver of child with 



congenital syphilis. Highly magnified. 



2. Parasitic Worms 



Thus far we have been considering causative agents of disease 

 which are popularly called microbes, and we must now turn from 

 this "world of the infinitely little" to somewhat larger organisms 

 which form a most important part of medical zoology. This field 

 may be illustrated by various kinds of parasitic worms. (Fig. 250.) 



Trematodes. There are many parasitic Flatworms, related to 

 the free-living Planaria, that comprise the group Trematoda. 

 Parasitic species exhibit, for the most part, complicated life his- 

 tories which have taxed the patience and ingenuity of biologists 

 to unravel. Among the numerous species, the Liver Fluke is per- 

 haps of most interest. (Figs. 43, 156, 161, 251.) 



