382 PHYSIOLOGY OF THE IN VERTEBRA TA. 



number of these feet are continually being retracted, while 

 others are being protruded, and this state of affairs goes on 

 alternately.* 



These animals can crawl up perpendicular surfaces, but 

 are very soon tired. This is due to the absence of any dif- 

 ferentiated structures in the form of sucking discs. 



The mode of locomotion of Astropedcn is peculiar. Upon 

 a dry, flat surface, it " points all the feet of all the rays in 

 the direction of advance, and then simultaneously distends 

 them with fluid ; they thus become so many pillars of support, 

 which raise the animal as high above the flat surface as their 

 own perpendicular length. The fluid is then suddenly with- 

 di-awn, and Astrupccten falls forward flat with a jerk. This 

 manoeuvre being again and again repeated at intervals of 

 about a quarter of a minute, the animal progresses in a uni- 

 form direction at the rate of about an inch per minute. In 

 this mode of progression, all the feet of all the rays are co- 

 ordinated in their action for determining one definite direction 

 of advance — those in the ray facing that direction acting 

 forwards, or centrifugally, those in the hinder rays backwards, 

 or centripetally, and those in the lateral rays sideways." 



When Astropedcn is walking along a flat horizontal surface 

 under water, its mode of locomotion is the same as the above, 

 only the motion is very rapid. " It appears, however, as if 

 the feet, besides being used as walking poles in the manner 

 just described, are also used to sweep backwards along the 

 floor of the tank, and so to assist in propelling the animal 

 forwards after the manner of cilia. Therefore, while walking 

 in water, Astropedcn is kept stilt-high above the surface on 

 which it is walking, by some of its feet, while others of its 

 feet are engaged in these sweeping movements." 



Astropedcn has a rapid rate of movement, being between 

 one and two feet per minute. When jjlaced upon its back, it 

 has the power of righting itself very rapidly. 



* The feet usually remain extended for a quarter to half a minute, but 

 very suddenly collapse. 



