PHYSIOLOGY 109 



appears to be condiictile as judged by the results of micro-dissec- 

 tion experiments of Taylor (p. 56). 



The Protozoa which possess myonemes are able to move by 

 contraction of the body or of stalk, and others combine this with 

 the secretion of mucous substance as was found in Haemogrega- 

 rina and Gregarinida. 



Irritability 



Under natural conditions, the Protozoa do not behave always 

 in the same manner, because several stimuli act upon them usu- 

 ally in combination and predominating stimulus or stimuli vary 

 under different circumstances. Many investigators have, up to 

 the present time, studied the reactions of various Protozoa to 

 external stimulations, full discussion of which is beyond the scope 

 of the present work. Here one or two examples in connection with 

 the reactions to each of the various stimuli will only be men- 

 tioned. Of various responses expressed by a protozoan against a 

 stimulus, movement is the most clearly recognizable one and, 

 therefore, free-swimming forms, particularly ciliates, have been 

 the favorite objects of study. We consider the reaction to a stimu- 

 lus in protozoans as the movement response, and this appears in 

 one of the two directions: namely, toward, or away from, the 

 source of the stimulus. Here we speak of positive or negative re- 

 action. In forms such as Amoeba, the external stimulation is first 

 received by the body surface and then by the whole protoplasmic 

 body. In flagellated or ciliated Protozoa, these processes act in 

 part sensory, in fact in a number of ciliates are found non-vibra- 

 tile ciha which appear to be sensory in function. In a compara- 

 tively small number of forms, there are sensory organellae such 

 as stigma (p. 79), ocellus (p. 80), statocysts (p. 77), concretion 

 vacuoles (p. 77), etc. 



In general, the reaction of a protozoan to any external stimulus 

 depends upon its intensity so that a certain chemical substance 

 may bring about entirely opposite reactions on the part of the 

 protozoans in different concentrations and, even under identical 

 conditions, different individuals of a given species may react dif- 

 ferently. 



Reaction to mechanical stimuli. One of the most common 

 stimuli a protozoan would encounter in the natural habitat is 

 that which comes from contact with a solid object. When an 

 amoeba which Jennings observed, came in contact with the end 



