3i6 PHYSIOLOGY OF THE INVERTEBRATA. 



grasping hold of sea-weeds, &c., when an Ecliinus is climbing 

 perpendicular or inclined surfaces of rock. 



Starfishes (with the exception of Brittle-stars) and Echini 

 are attracted by light, but when their eye-spots are removed, 

 they no longer are so. Romanes and Ewart have demon- 

 strated that severing the ray-nerve destroys all physiological 

 continuity between the pedicels on either side of the division. 

 Severing the nerve at the origin of each ray, or severing the 





■f 



Fig. 6i. — Nervous System of Starfish. 

 a = nerve-ring. I: = ambulacral nerve, c = eyes. 



nerve-ring (Fig. 6i) between each ray, has the effect of 

 totally destroying all co-ordination among the rays ; " there- 

 fore the animal can no longer crawl away from injuries, and 

 when inverted it forms no definite plan for righting itself — 

 each ray acting for itself without reference to the others, 

 there is, as a result, a promiscuous distribution of spirals and 

 doublings, which as often as not are acting in antagonism to 

 one another. This division of the nerves, although so com- 



