OBSERVATIONS ON THYONE BRIAREUS. 265 



it away from the source of the h'ght, as Tliyonc is very sensitive 

 to photic stimulation. Locomotion is brought about by shorten- 

 ing the tube-feet after they have been extended and attached, by 

 twisting and extending movements of the whole body, and it is 

 also assisted by sharp waves of muscular contraction which travel 

 from one end of the body to the other. 



The tube-feet act by pulling. They were never observed to 

 become rigid enough to lift the body from the surface on which 

 it rested, nor was there any pushing action, such as Jennings 

 (:07, p. 99) described in the starfish. There was some lack of 

 correlation in their movements and this manifested itself in two 

 ways. When locomotion was taking place in a definite direction, 

 the tube-feet were not only extended on the side towards which 

 the animal was moving, but also over all the rest of the body. 

 This was doubtless due partly to the fact that the tube-feet serve 

 as organs of touch as well as of locomotion, but there were 

 nevertheless a large number of seeking movements which were 

 apparently of no use in locomotion. Furthermore, some of the 

 tube-feet which were behind as an individual moved often 

 remained attached for some time after they could be of any help 

 in locomotion, and, after being greatly stretched, they were 

 actually jerked from their attachments with a snap. They were 

 never torn loose from the body, however, as often happens when 

 Arbacia is pulled away from a solid surface. The stimulation 

 which brings about the attachment of the terminal discs of the 

 tube-feet is apparently contact with a solid object. Bits of shell, 

 sand, and other bodies were frequently held by them for several 

 days at a time. 



In addition to the pulling action of the tube-feet, locomotion 

 was often assisted by movements of the body. Individuals some- 

 times assumed a shortened form, detached the tube-feet at one 

 end of the body, and then elongated this free end or made slow 

 seeking movements which were somewhat like those of a leech. 

 This free portion of the body was then attached and the animal 

 slowly regained its contracted form again, thus making some 

 progress. The sharply defined waves of contraction which com- 

 monly passed from one end of the body to the other were ap- 

 parently of use chiefly in enabling the tube-feet to gain a new 



