INTERNAL DISEASES 403 



to man, dogs and cats; Bothriocephalus cristatus, which is 

 seldom found in man, and is commonest in dogs in Northern 

 Greenland. 



Besides these worms, certain unicellular organisms are not 

 infrequently observed as internal parasites in man. Thus 

 among the Infusoria, Cercomonas intestinalis is found in the 

 intestine in cases of cholera and diarrhcea (Plate VI., fig. 7); 

 Trichomonas intestinalis in acute and chronic diarrhcea and also 

 in typhoid (Plate VI., fig. 8); Balantidium coli in severe 

 chronic affections of the intestine (Plate VII., fig. 8) ; Coccidium 

 oviforme, a sporozoon, is often transferred to man from rodents, 

 such as mice and rabbits. Amceba (A. coli) has been recog- 

 nised by Lo'sch in the large intestine in men suffering from 

 dysentery. 



Very few external parasites are peculiar to man ; the flea, 

 Pulex irritans (Fig. 201) ; the head-louse, Pediculus capitis (Fig. 



FIG. 202. FIG. 203. FIG. 204. 



Human head-louse. Crab-louse. Bed bug. 



202), found, according to Darwin, on various races in different 

 parts of the world ; the clothes-louse, Pediculus vestimentorum ; 

 the crab-louse, Pediculus pubis (Fig. 203) ; and the bed bug, 

 Cimex lectuarius (Fig. 204). 



Many other sorts of fleas may be transferred to man from 

 dogs, cats, etc, but they soon leave his body. 



Parasites not peculiar to man but found also upon other 

 animals are the sand flea, Pulex penetrans, the Sarcoptes or 

 Acarus scabiei (causing itch), which according to Ziegler also 

 lives on horses and some sorts of sheep ; the Macrogaster 

 folliculorum, which is parasitic on the skin of man and of various 

 animals (dogs, cats, swine, cattle and goats) ; and the sheep tick, 

 Ixodes ricinus, which occasionally occurs as an unwelcome 

 parasite on man, though common on dogs, cattle, sheep and 

 wild animals. 



Among vegetable parasites only three are undoubtedly 



26 * 



