ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 465 



terminal suckers into action upon the floor of the tank, and then by a 

 successive and similar action of the suckers further back in the series 

 the whole ray is progressively twisted round." Spatangus uses its 

 longer spines to push and prop itself. 



Stimulation causes them (1) to escape from injury in a direct line 

 from the source of irritation ; (2) in a diagonal between two points ; 

 (3 ) there is a marked rotation on the vertical axis when several points 

 all round the animal are simultaneously stimulated ; (4) if a circular 

 band of injury be made round the equator the animal crawls away 

 from the broadest part of the band. When any part of the external 

 surface is touched, the pedicels, spines, or pedicellariae within reach 

 close on the point ; the pedicellarise are the most active, and if they 

 can they seize the body by their forceps ; this observation leads iis 

 to see the function of the pedicellari^. When climbing perpendicular 

 surfaces of rock covered with waving seaweed an immediate hold is 

 obtained, and the weed is held steady till the pedicels affix to it their 

 suckiug-disks ; this last operation takes about two minutes, and this 

 is about the time that the forceps are able to retain their grasp. 



The spines bristle when the surface of an echinus is severely 

 irritated ; the pedicels of a given row are retracted when irritation is 

 applied to that row. Irritation of the external plexus stimulates the 

 pedicels, but the former, or inhibitory, action is the stronger of the 

 two. Unless the eye-spots are removed, Asterids and Echinids crawl 

 towards light. 



Single rays crawl as fast and as regularly as entire animals, are 

 affected by light, crawl up perpendicular surfaces, and are able to 

 right themselves. When the nerve-ring is severed at its centre, the 

 rays act independently ; but this mode of section does not seem to 

 affect the physiological continuity of the external nerve-j)lexus, which 

 would seem to connect with one another all the pedicels and muscles 

 without any reference to the main trunks. Injury of the external 

 plexus destroyed " the nervous connections on which the spines and 

 pedicellarisB depend for their function of localizing and closing 

 round a seat of stimulation," but it does not destroy that co-ordination 

 of the spines which is necessary for locomotion ; the seat of this 

 influence would appear to lie in a not yet observed internal plexus. 

 A proof from the physiological point of view is found in experiments 

 by which the whole of the internal surface of the test is completely 

 cleaned ; after this the spines and pedicellarise shortly become perfectly 

 quiescent, to feebly recover themselves after a few hours. This " hypo- 

 thetical internal plexus '' would appear to be in close connection with 

 the external plexus. 



Echini may be divided, and the pieces, however small, still exhibit 

 local reflex irritability. In the righting action the pedicels would 

 appear to act serially, and to be assisted by nervous co-ordination. 

 Removal of the pentagonal nerve-ring has no effect on the pedicellarifB 

 or on the local reflex action of the spines, though the general co- 

 ordination of these latter is completely destroyed : this nerve-ring has, 

 in effect, the function of a nerve-centre. And, on the whole, it is 

 found that the " nervous system is in function, as in structure, every- 



