THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 275 



An Antedon when once attached exhibits very little tendency to 

 alter its position, and may remain fixed in the same place for weeks. 

 If detached, either spontaneously or by force, it can, and usually does, 

 swim actively until it reaches a suitable place of rest, to which it 

 anchors itself by its cirri. The normal swimming movements, which 

 are peculiarly graceful, consist in strong flexion of the proximal half 

 of the arm, which is raised vertically over the disc, and then extension 

 of the whole arm, the distal half of which is thrown out something 

 like a whiplash or the line of a flyrod. During flexion the pinnules 

 are folded alongside the arm ; during extension spread out so as to 

 expose as great a surface as possible. Usually two or three arms are 

 raised simultaneously, sometimes as many as five, and the only rule I 

 have noticed is that the two arms of each pair are always flexed 

 alternately and not simultaneously. 



When attached by its cirri, the arms of Antedon exhibit but very 

 slight movements ; they are usually spread out widely, apparently to 

 expose as large a surface as possible for the entanglement of food 

 particles, which, if they once come in contact with the ambulacral 

 epithelium, get carried by the action of its cilia to the mouth. 



Irritation of the ambulacral groove at any part causes the adjacent 

 pinnules to be at once turned forwards, i.e. with their tips towards 

 the free end of the arm, and folded alongside the irritated part, 

 apparently to protect it from further injury. Slight irritation of a 

 pinnule or of an arm causes correspondingly slight and local move- 

 ments ; stronger irritation causes movements of the whole arm, which 

 may spread to other arms, or lead to the animal detaching itself and 

 swimming freely. Irritation of the oral pinnules, however slight, 

 causes them to be firmly closed over the disc, and stronger or prolonged 

 irritation causes the arms to be flexed strongly, so as to cover the disc, 

 or else the whole animal to detach itself and swim away. 



If an Antedon be detached and placed with its oral surface down- 

 wards, it will right itself almost at once. If the surface on which it 

 is placed be a rough one, the righting movement is effected in a few 

 seconds or almost instantaneously. In a glass vessel it takes longer 

 to perform, but with an active specimen I have never seen more than 

 two minutes spent over the operation. In righting itself an Antedon 

 first flexes all the arms slightly, so as to raise the disc a little above 

 the ground ; then follows a moment of apparent uncertainty as to 

 which arm to use. One arm is then flexed more strongly than the 



