STAR- FISH AND SEA-URCHINS. 817 



nerve-ling is that of entiicly destroying the ability 

 to peiforni this manoRiivre in the case of the majority 

 of specimens ; nevertheless about one in ten continue 

 able to perform it. Again, if an Echinus is divided 

 into two hemispheres by an incision carried from pole 

 to pole through any meridian, the two hemispheres 

 wiU live for days, crawling about in the same 

 manner as entire animals; if their ocular plates 

 are not injured, they seek the light, and when 

 inverted they right themselves. The same observa- 

 tions apply to smaller segments, and even to single 

 detaclied rows of ambulacral feet. The latter are, 

 of course, analogous to the single detached rays of 

 a Star-fish, so far as the S3^stem of ambulacral feet 

 is concerned ; but, looking to the more complicated 

 apparatus of locomotion (spines and pedicellariye), 

 as well as to the rigid consistence and awkward 

 shape of the segment — standing erect, instead of 

 lying flat — the appearance presented by such a seg- 

 ment in locomotion is much more curious, if not 

 surprising, than that presented by the analogous 

 part of a Star-fish under similar circumstances. It 

 is still more surprising that such a fifth-part seg- 

 ment of an Echinus will, when proj'jped up on its 

 ab-oral pole (Fig. 62), right itself (Fig. 03) after 

 the manner of larger segments or entire animals. 

 They, however, experience more difficulty in doing 

 so, and very often, or indeed generally, fail to 

 coujplete the manoeuvre. 



On the whole, then, we may conclude that the 

 nervous system of an Echinus consists (1) of an 

 external plexus which serves to unite all the feet, 



