SECT. VI. 



ECHINODERMA TA. 



179 



Fig. 139. Sucker-plate of 

 Sea-Egg. 



seein to possess except that of touch and probably 

 smell. The nervous system is a slender, equal- sided 

 pentagon round the gullet, from the sides of which five 



nerves are sent to the muscles of 



the mouth, and others, extending 

 along the ambulacral or feet bands, 

 end in nerve-centres under the 

 eye-specks. 



The mechanism for extending 

 and retracting the feet by a liquid, 

 is the same with that in the star- 

 fishes, but the pores which admit 

 the liquid into the feet are double. 

 The tubular feet swell at their 

 extremity into a fleshy sucker, within which there is a 

 thin glassy reticulated rosette (fig. 139), of which fig. 

 140 is a highly mag- 

 nified segment. It 

 is perforated in the 

 centre by a large 

 round opening. The 

 sea-urchins can 

 stretch their feet 

 beyond the spines, 

 and by means of the 

 suckers they can at- 

 tach themselves even 



,, , . Fig. 140. Section of a sucker-plate. 



to smooth objects, or 



aided and directed by their spines they roll themselves 



along with a rotatory motion head downwards. 



The circulation of the bright yellow blood is like that 

 of the star-fishes. It is aerated both internally and 

 externally. The external respiratory organs are short, 

 branched, and highly vibratile bodies attached in pairs to 

 oval extremities of the fine imperforated bands. 



N 2 



