THE STRUCTURE AND FUNCTIONS OF NERVE-TISSUE. 29 



the bases of the five rays, into which they send, and from 

 which they receive, nerve-fibres ; the ganglia are likewise 

 connected with one another by a pentagonal ring of fibres. 

 Now experiment shows that in this simple, and indeed geo- 

 metrical plan of a nervous system, the constituent parts are 

 able, when isolated by section, to preside over the movements 

 of their respective muscles ; for if a single ray be cut off at 

 its base, it will behave in all respects just like the entire star- 

 fish — crawling away from injury, towards light, up perpen- 

 dicular surfaces, and righting itself when turned upon its 

 back. That is to say, the single nerve-centre at the base of 

 a single separated ray is able to do for that ray what the 

 entire pentagonal ring, or central nervous system, is able to 

 do for the entire animal ; it is for that ray the trigger which, 

 when touched by the advent of a stimulus, tlirows the mus- 

 cular mechanism into appropriate action. Thus it is evident 

 that each of the five nerve-centres stands in such anatomical 

 relation to the muscles of its own ray, that when certain 

 stimuli fall upon the ray, the process of reflex action leaves 

 no choice of response. The beauty and delicacy of this 

 mechanism is shown when in the u.nmutilated animal all the 

 nerve-centres are in communication as one compound nerve- 

 centre. For now, if one ray is irritated, all the rays will 

 co-operate in making the animal crawl away from the source 

 of irritation; if two opposite rays are simultaneously irri- 

 tated, the star-fish will crawl away in a direction at right 

 angles to an imaginary line joining the two points of irrita- 

 tion. And, more prettily still, in the globular Echinus, or 

 sea-urchin (which is, anatomically considered, a star-fish 

 whose five rays have become doubled over in the form of an 

 orange, soldered together and calcareous so as to make a 

 rigid box), if two equal stimuli be applied simultaneously at 

 any two points of the globe, the direction of escape will be 

 the diagonal between them ; if a number of points be simul- 

 taneously irritated, one effect neutralizes the other, and the 

 animal rotates upon its vertical axis ; if a continuous zone of 

 injury be made all the way round the equator, the same thing 

 happens; but if the zone be made wider at one hemisphere 

 than the other, the animal will crawl away from the greatest 

 amount of injury. So that in the Echinoderms the geometrical 

 distribution of the nervous system admits of our making ex- 

 periments in reflex action with very precise quantitative 



