PHYSIOLOGY OF NERVOUS SYSTEM 



523 



the stored tone spreads upwards to the spines, causing the weak 

 form of spine reaction, and the spines converge. 



It will be seen therefore that the so-called central nervous 

 system of Echinus does not act in any sense as a brain, as indeed 

 might have been guessed from the absence of any differentiation 

 in it. As Uexklill points out, when an animal is covered all 

 over with similar organs, such as spines and pedicellariae, 

 capable of acting automatically, a brain is not needed. The 

 object of a brain is to direct organs which are in a certain place 



t. 



Fig. 233. To show character and distribution of the sphaeridia in Strongylocentrotus 



droebachiensis. A, a portion of a radius, with sphaeridia, and the adjoining edge 

 of the peristome, p, Pair of pores for a tube-foot ; per, peristome ; t, primary 

 tubercle. B, an isolated sphaeridiiim. (After Loven.) 



to a danger which may come from any quarter, but in the Sea- 

 urchin any spine is as good as any other spine, and such orienta- 

 tion is not needed. " In a dog the animal moves its legs, in a 

 Sea-urchin the legs move the animal." What the Sea-urchin 

 does need is a means to prevent its pedicellariae attacking its 

 own organs with which they may come into contact. Thus it 

 possesses an " autodermin," a chemical contained in the ectoderm 

 which paralyses the muscles of the pedicellariae, as may be seen 

 by offering to them a spine of the same animal. If, however, 

 the spine be treated with boiling water, and then offered, it is 

 viciously seized, showing that this substance can be dissolved out. 

 Just as in the case of the Starfish, when the nerve-ring is 



