362 The Animal Kingdom 



long firm pseudopodia known as axopods. These extend from around 

 the cell body, suggesting the rays of a stylized sun. 



Among the marine forms is the order Foraminijera (Fig. \\S,B) 

 whose members have beautifully marked shells. The shells of these 

 forms are found in tremendous numbers in various marine deposits 

 such as chalk beds and cliffs. They have been studied extensively by 

 the oil geologists as various genera give indication of oil-bearing strata. 

 The siliceous shells of the order Radiolaria (Fig. 115,£) form large 

 deposits on the ocean bottoms, the so-called radiolarian ooze. 



Parasitic Sarcodina. — A number of parasitic amoebae are known. 

 Species of these occur both in man and in other animals. In human 

 beings Endamoeha coli occurs in the colon, E. gingivalis in the mouth, 

 and several other species in the intestine. Most of these appear to be 

 relatively harmless to their hosts. However, one, E. histolytica (Fig. 

 115,F and G), causes amoebic dysentery. These amoebae invade the 

 epithelial cells of the intestinal tract, causing abscesses. If the in- 

 fection is not arrested in the earlier stages, the amoebae may migrate to 

 the heart, liver, lungs, spleen, or brain. In these organs they likewise 

 form abscesses, and fatalities may result. 



In the host, E. histolytica produces numerous individuals by re- 

 peated binary divisions. Among these individuals will be found many 

 precystic forms. Each precystic form rounds up to produce a cyst 

 with four nuclei. The cyst passes out with the feces. If these cysts are 

 ingested by another individual, they divide to produce eight uninucleate 

 amoebae and a new infection is established. From this short account 

 of the life history it can be seen that infection with E. histolytica is 

 always due to carelessness in the handling of foods, water, or in dis- 

 posal of feces. The cysts can also be transmitted by flies. 



From various observations, it is concluded that about 10 per cent 

 of the world's population is infected with E. histolytica. Some strains 

 are more virulent than others, and some individuals appear to be more 

 resistant to the parasite. These resistant individuals may carry the 

 infection for years without showing ill effects, but if they are careless 

 in handling food, they may produce foci of infection. These individuals 

 are known as "carriers." Of course, their infection may be detected 

 by fecal examination as both the cysts and the active stages are recog- 

 nizable. 



