ECHINOIDEA. 263 
cushion-like, flat, or obviously bilateral; the test ts covered 
with spines which vary greatly in length and thickness in the 
different types’; the locomotor and respiratory tube-feet usually 
extend from the peristome to near the aboral pole; there is 
often a well-developed system of apical plates ; the mouth is at 
the lower pole, the anus either at the aboral pole or in the 
posterior inter-radius ; the gonads are unpaired, five in 
number, and inter-radial. 
Description of the Common Sea-Urchin. 
Most sea-urchins live off rocky coasts, and not a few 
shelter themselves sluggishly in holes. They move by 
Fic. 136.—Apical disc of sea-urchin 
In the centre is the periproct bearing the anus; around it are five 
genital or inter-radial plates (g.), one of which is modified as the 
madreporite (7.); beyond these five ocular or radial plates (0.) ; 
z.y.a., an inter-radial or inter-ambulacral area, with spines 
only ; ~.@., a radial or ambulacral area, with spines and open- 
ings for tube-feet. 
means of their tube-feet and spines, and seem to feed on 
“acorn-shells” and other small sedentary animals, some 
seaweeds, and the organic matter found in ‘mud and other 
