ECHINODERMATA — SEA URCHINS 



167 



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amb.. 



Fig. 66. — The common sea urchin, Slrongylocentrotus drobachiensis Say, from the 

 coast of Maine. Entire individual, dorsal view (with spines, etc., removed from 

 half of the shell (test)). Below is the central dorsal area with ornamentation 

 omitted, a., anus, the large black spot within the periproct (the area within the 

 circle of genital plates and protected by many small plates) ; amb., ambulacrum ; 

 g., genital plate at tip of each interambulacrum, perforated by the genital opening, 

 — the large black spot; int. amb., interambulacrum; mad., madreporite; oc, 

 ocular plate at tip of each ambulacrum. 



