ECHINO1DEA. 



263 



1 cushion-like, flat^ or obviously bilateral ; the test is covered 

 ivith spines which vary greatly in length and thickness in the 

 different types ; the locomotor and respiratory tube-feet usually 

 extend from the peris tome 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^ Jive 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 



f 



FIG. 136. Apical disc of sea-urchin 



In the centre is the periproct bearing the anus ; around it are five 

 genital or inter-radial plates (-.)> one of which is modified as the 

 madreporite (m.) ; beyond these five ocular or radial plates (0.) ; 

 i.r.a., an inter-radial or inter-ambulacral area, with spines 

 only; r.a., 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 



