CHAFTER X. 



STAR-FISH AND SEA-UIICHINS. 



Structure of Star-fish and Sea- Urchins. 



We shall now proceed to consider in tlie organization 

 of the Echinodermata a type of nervous system 

 which is more highly developed than that of the 

 Medusre. 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 

 disc and five radiating arms (Fig. 82 \ Upon the 

 whole of the upper surface there occur numerous 

 calcareous nodules embedded in the soft tiesh, and 

 supporting short spines. One of these nodules is 

 much larger than any of the others, is constant in 



