164 UNICELLULAR ANIMALS. 



(1) Contractility, by means of which motion is effected. 

 This appears most clearly when the animal is stimulated by a 

 sudden jar, or by an electric shock, which causes the body to 

 contract into a ball. This property, precisely like the contraction 

 of a muscle (p. 27), is the result of a molecular rearrangement, 

 accompanied by chemical changes, which causes a change of 

 form in the mass without altering its bulk. The action of the 

 contractile vacuole is due to the contractility of the surrounding 

 protoplasm ; and in like manner the currents which cause the 

 protrusion and withdrawal of pseudopods, and so the locomotion 

 of the animal as a whole, are produced by localized contractions 

 of the peripheral layer of protoplasm which drive onwards the 

 more fluid central parts. 



(2) Irritability (including Co-ordination), or the power to 

 be affected by, and to respond to, changes or " stimuli" acting 

 upon or within the protoplasm. The change of shape following 

 the application of an electric shock is actually effected by con- 

 tractility, but the power to be affected by the shock and to arouse 

 contractility, is irritability. To this property the animal owes its 

 power of performing adaptive actions in response to changes in 

 the environment, and also its power to co-ordinate the various 

 actions of its own body. To illustrate : It is a remarkable fact 

 that Amoeba is able to discriminate between nutritious and innu- 

 tritions matters, ingulfing the former, but rejecting the latter. 

 Physiologically this discrimination is a difference of response to 

 different stimuli hence a phenomenon of irritability. Again, 

 the various actions (movements, etc.) of Amoeba, despite their 

 apparently vague character, are co-ordinated to form a definite 

 whole ; and co-ordination may be regarded as a phenomenon of 

 irritability, changes in one part serving as stimuli to other parts 

 and being brought into orderly relation with them. The property 

 of irritability lies at the base of all nervous activity in higher 

 forms (cf. p. 67) and is concerned in many other actions. 



(3) Metabolism, the most fundamental of all vital actions, 

 since it lies at the root of all, is the power of waste and repair 

 the destructive chemical changes in protoplasm (Jfatabolism) 

 whereby energy is set free, and the constructive actions (anabo- 

 lism) through which new protoplasm is built and potential 

 energy is stored (cf. p. 33). There is every reason to believe 



