142 ANIMAL BEHAVIOUR 



52. ON TAXIS 



By taxis, or the tactic movements of animals, is meant move- 

 ments of the whole animal effected in response to a vector 

 stimulus. Examples are (i) the swimming movements of many 

 larvae with respect to the direction and intensity of the incident 

 light : a very good example is afforded by opening the shells 

 of Barnacles {Balaniis) about the end of March and placing the 

 embryos in a saucer of sea water standing near to a lamp. As 

 the larvae hatch out they swim along the surface of the water 

 towards the light and then away from the light at the bottom 

 of the water. This is phototaxis. (2) If a capillary glass tube 

 be filled with a culture of some kinds of aerobic bacilli and then 

 observed beneath the microscope it may be found that the 

 organisms will move towards the open ends, where the oxygen- 

 concentration will be greatest. If the ova and spermatozoa of 

 many marine animals (say those of a flounder) be placed in water 

 the spermatozoa will be seen to move towards, and attach them- 

 selves to the ova. These are examples of chemiotaxis. (3) If 

 a current of electricity be passed through water containing some 

 kinds of organisms it may be found that the latter will move 

 either with or against the current. This is galvanotaxis . 



As in the case of tropisms the taxis has sign and this may 

 undergo reversal when the direction of the stimulus changes. 

 And when the tensor of the stimulus changes the sign of the 

 taxis may also change. 



Taxis constitutes true behaviour since the activities of the 

 animal, as a whole, are involved. There is also purpose in our 

 sense. For instance, the oxygen-concentration in the capillary 

 tube mentioned above is lowered by the respiration of the bacilli 

 and the assimilation, or other activities of the latter will tend to 

 fall off. Therefore they move to the neighbourhood of the 

 open ends where oxygen is being taken up from the outer 

 medium. Thus normality is maintained. But it does not seem 

 possible to ascribe purpose, in this way, to all so-called tactic 

 behaviour. 



^za. The Resolution of Taxis. It is possible, in many 

 cases, to explain the mechanism of taxis. 



(i) There is a basis of random movements. The animal 

 *' fidgets " because it is continually being stimulated by small 



