10 



PLANT-ANIMALS 



[CH. 



this area it acts as a stimulus to the nervous tissues 

 beneath. As the result of stimulation of this tissue, 

 nervous impulses may be despatched to the muscles 

 of the body, and, causing them to contract, give rise 

 to movements of the body which are definite in 

 direction. 



Thus the otocyst serves as an indicator of the 

 line of gravity ; in other words it acts as the organ 



oc 



Fig. 3. Young Convoluta paradoxa. C = cilia covering the surface 

 of the bod}% OT = otocyst. OC = eyes. V = empty digestive 

 vacuoles. 



for gravi-perception. By its means, the animal is able 

 to orientate itself with respect to the vertical, and 

 so to find its way downward or upward. 



That the otocyst does indeed serve this end 

 has been established by experiments with other 

 animals, and may be inferred in the case of C. ros- 



