FORMS OF BEHAVIOR IN ANIMALS 319 



been used in so many senses that it is meaningless, and the study 

 of animal behavior would be clarified if it could be ehminated. 

 If the word could have been restricted to the protozoa, it would 

 be convenient to use for the responses of unicellular organisms. 

 The protozoa respond to a variety of stimuli and thus exhibit an 

 obvious irritabiHty; and the protozoan cell responds as a whole, 

 since it is an independent organism. By contrast, the irritability 

 of metazoan cells is less diversified, because of their unbalanced 

 physiological state (c/. p. 128). We may, therefore, ignore the 

 term tropism and say that the protozoa exhibit irritability or 

 response to stimulation to an extent that is surprising. The trial- 

 and-error responses of ciliates are examples. 



Behavior in Animals like Hydra. — The existence of a nervous 

 system in the Coelenterata makes possible more effective response 

 to stimulation than occurs in sponges, although there is a similar 

 diffuse spreading of the reaction from the original point of stimu- 

 lation. Such animals are said not to have reflexes, because there 

 is no evidence of the passage of a nerve impulse in and out along an 

 estabhshed reflex arc {cf. p. 107). There is, however, neural 

 transmission in contrast with the neuroid transmission in sponges. 

 Such a condition as exists in the hydra might be termed a yre- 

 reflex type of response. From this, reflexes might be evolved by 

 the estabhsment of definite paths in a more centralized nervous 

 system, as in the earthworm. 



Behavior in the Earthworm and in Higher Animals. — That 

 reflex action occurs in animals like the earthworm and other 

 Annulata may be inferred from the fact that the individual reacts 

 as though the stimulus were " reflected " in the response. Such 

 a conclusion from the visible effects that follow stimulation is 

 supported by the existence of a reflex arc as pictured by the recep- 

 tor-adjustor-effector mechanism (Fig. 144, p. 298). The behavior 

 of such animals is composed of reflex actions, modified and com- 

 pounded to some extent as in higher animals. The principal dif- 

 ference between the behavior of the earthworm and that of higher 

 metameric invertebrates, such as crustaceans and insects, consists 

 in complexity of reflexes and the modifiability of behavior bj^ what 

 is called memory, or the impress of past experiences, in spite of the 

 high development of instincts. In the chapter on irritability, it 

 was shown that instincts may be defined as inherited reflexes. The 

 manner in which instincts grade into forms of behavior that are 



