CHAPTER X. 
STAR-FISH AND SEA-URCHINS. 
Structure of Star-fish and Sea-Urchins. 
WE shall now proceed to consider in the organization 
of the Echinodermata a type of nervous system 
which is more highly developed than that of the 
Medusee. In conducting this research, I was joined 
by my friend Professor J. Cossar Ewart, to whose 
unusual skill and untiring patience the anatomical 
part of the inquiry is due. But here, as formerly, 
I shall devote myself to the physiology of the sub- 
ject, as it is not possible within the limits assigned 
to this volume to travel further into morphology 
than is necessary for the purpose of rendering the 
experiments intelligible. I shall therefore begin by 
seeking to give merely such a general idea of the 
structure of the Echinodermata as is necessary for 
this purpose. | 
As we all know, a Star-fish consists of a central 
dise and five radiating arms (Fig. 32). Upon the 
whole of the upper surface there occur numerous 
calcareous nodules embedded in the soft flesh, and 
supporting short spines. One of these nodules is 
much larger than any of the others, is constant in 
