90 ONE-CELLED ANIMALS— PHYLUM PROTOZOA 



pass through unfavorable periods, such as the drying up of a pond, by 

 developing a protective coating or cyst around themselves and reducing 

 metabolism to the barest minimum necessary to maintain life. In this 

 encysted form they are able to withstand such unfavorable periods and 

 return to the vegetative state when conditions become normal again. 



Amoebae display irritability even though they have no distinguish- 

 able areas of sensitivity. They will move away from a strong light 

 source; this can be seen by viewing them under the microscope while 

 shining a light from one side of the slide. Also, it is demonstrated by 

 the fact that they are most abundant in the darkest portions of labora- 

 tory culture jars. They will react to various chemicals, moving to- 

 ward some that attract them and away from others that repel them. 

 They can distinguish between food material and other matter in the 

 water ; they engulf only that which they can use as food. They will 

 actively pursue small water animals in an effort to capture them for 

 food. They respond to heat and cold, always moving to the area of 

 the water which is nearest their optimum temperature. They respond 

 to vibrations in the water by pulling in extended pseudopodia. In these 

 various reactions to their environment these single cells display sensory 

 perceptions akin to those of the highest forms of animal life. Senses 

 similar to touch, taste, smell, sight, temperature sensitivity, and even 

 hearing are represented in these perceptions. 



There is another genus in the Sarcodina which is amoeba-like in its body 

 form. This is the genus Endamoeba which is so named because it lives 

 within the body of other forms of life. The first species of the genus that 

 we will consider is Endamoeba coli, which lives in the body of man and 

 other vertebrates. This organism may be found in the colon (large in- 

 testine) of about half of all people, but fortunately it is harmless, since it 

 feeds on the indigestible food or feces which passes through the colon. 

 It is considerably smaller in size than Amoeba proteus and somewhat more 

 sluggish in its reactions. 



A closely related species, Endamoeba histolytica, has developed the 

 habit of attacking the lining of the intestinal wall, and thus it becomes a 

 serious parasite of man. When these organisms are in the intestine of 

 man, they attach themselves to the cells and secrete a tissue-dissolving en- 

 zyme that destroys the cells and permits the Endamoebae to go deeper into 

 the wall of the intestine and destroy more cells, and so on until they may 

 produce a serious ulceration of the colon. In extreme cases this may cause 

 perforation of the wall of the colon, liberating millions of bacteria, which 

 are always present in a human colon. These pass out into the body cavity 

 and set up an infection known as peritonitis which may result in death. 

 Fortunately, there is a tough muscular layer in the intestine that normally 



