NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 49 



but much more active spatangoid Plagiobrissus grandis, which can 

 bury itself in a little over 10 miuutes. 



Meoma ventricosa is able to right itself relatively rapidly when 

 overturned. An excavated specimen was placed on the sand on its 

 dorsal surface, along with three specimens of Clypeaster subdepressus, 

 two of which are illustrated on plate 6, figures 1-6. The process of 

 righting began at 10:15. By 10:30 M. ventricosa had achieved a 

 strong tilt while two of the C. subdepressus were barely moved, and 

 one small one slightly tilted. The M. ventricosa was nearly on edge 

 by 10:35, but the most rapid C. subdepressus (pi. 6, fig. 2) did not 

 attain an angle of 45 degrees until 10:50. The M. ventricosa had 

 righted itself by 10:40, in contrast, the most rapid of the C. subde- 

 pressus did not become completely righted until about 11 :15, but then 

 was completely buried and actively burrowing only three minutes 

 later. At that time the slower two C. subdepressus nearby had only 

 raised to about 45 degrees, when the observations were terminated. 



Meoma ventricosa turns over on its anterolateral edge, in a direc- 

 tion similar to that of C. subdepressus, and in contrast to the anterior 

 direction of overturning of Encope michelini and Leodia sexie$per- 

 forata. However, it does not dig that edge into the sand as does C. 

 subdepressus, but seems merely to "walk" itself over on its spines, 

 while remaining on the surface of the sand. 



This species apparently feeds by passing sand through its system 

 and extracting whatever nutrient particles are included. A dissected 

 specimen 144 mm. in length was nearly entirely full of sand ; the en- 

 tire contents of the test had a dry weight of 191 grams. The mouth 

 remains open as the animal moves anteriorly through the sand, the 

 labrum serves as a scoop, and sand is forced into the mouth by the 

 anterior motion of the whole animal. 



Predation. — Meoma ventricosa seems to be rather frequently vic- 

 timized by the starfish Oreaster reticulatus (Linnaeus). One such 

 incident was observed directly (pi. 12, figs. 1, 2). The urchin was 

 unburied, on the surface of the sand, and the starfish was draped over 

 its dorsal surface. When the starfish was lifted off, its extruded gut 

 was seen to retract. The area of the urchin that had been covered by 

 the gut of the star was devoid of spines, and the test was compara- 

 tively thin (pi. 13, figs. 1-3) demonstrating local digestive dissolution. 

 As soon as the starfish was removed, the urchin began to move its 

 spines in the normal burrowing or reburying motion, but while the 

 starfish was on it, the urchin did not move. Possibly the starfish 

 secreted some form of mild narcotic agent along with whatever fluids 



