UNICELLULAR ORGANISMS 57 



water, and some forms of protoplasm much more, certain 

 organisms containing over 95%. When dormant, protoplasm 

 may remain alive with a far smaller percentage, dried seeds 

 containing as little as 8%. Some animals also may be dried 

 (dessicated) and still retain their vitality for a long time. This 

 is true of many of the microscopic, unicellular animals and also 

 of some of the higher types (e. g., Hydatina; see Fig. 116). In 

 all such cases life activities are suspended but will be resumed 

 when the animal imbibes water. 



Irritability. The Amoeba has no sense organs nor does it 

 have any nervous system. It is difficult or impossible to deter- 

 mine positively whether it has any conscious sensations, but 

 it certainly has the power of reacting when stimulated, thus 

 showing that it possesses irritability. 



Reaction to contact (Thigmotropism) (Gr. thigma = touch + 

 trope = a turning). If the moving Amoeba is touched by a solid 

 object, the part touched draws away from the object, new 

 pseudopodia being thrust out in another direction. If, however, 

 the object be a^particle of food, the animal is differently affected 

 and the pseudopodia flow around it so as to engulf it. 



Reaction to chemicals (Chemotropism) (Gr. chemesa = chemis- 

 try + trope ) . If certain chemicals are brought in contact 

 with the Amoeba, it moves off in some other direction. Sugar, 

 lactic acid, sodium chloride, and many other substances have 

 this effect. 



Reaction to heat (Thermotropism) (Gr. thermos = heat + trope). 

 The activities of the Amoeba are directly dependent upon tem- 

 perature. At a temperature of freezing, no activities are mani- 

 fest. If the temperature is raised the activities begin and become 

 more active with the increase in temperature up to a certain 

 point, about 85 F. If warmed still more, they become less 

 active, and when heated to about 90 F. the activities cease en- 

 tirely. At about 105 F. the protoplasm is coagulated and the 

 animal killed. If a warm or hot object is brought near an 

 active Amoeba the animal moves away from it. 



