NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 3~ 



outward to the sandy terraces beyond the main reef. It occurred in 

 depths from 12 to 85 feet, although only dead tests were found at the 

 greater depths. It was present only in relatively deep sand, not in the 

 shallow basins of sand between reef spurs, or patches within reef 

 masses. 



This species was found at stations 17 (dead), 17a (dead), 20 

 (dead), 23, and 30. 



Behavior. — Plagiobrissus grandis burrows through the sand, buried 

 just beneath the surface or as deeply as 2 inches. The long spines on 

 its dorsal surface lie back as the animal burrows anteriorly, but stand 

 straighter with the tips reaching the surface of the sand when for- 

 ward motion ceases. The trail left by this species is less conspicuous 

 than that of Meoma ventricosa, because it burrows deeper than that 

 species. However, weak trails were visible, although normally mis- 

 taken for less fresh trails of M. ventricosa. Living specimens were 

 discovered by digging at the ends of these trails, normally with the 

 expectation of finding M. ventricosa. 



When excavated, P. grandis moved rapidly over the surface of the 

 sand with a zig-zag or "fishtailing" motion, at a speed of about 1 foot 

 in 3 seconds. After moving about 3 feet away from an antagonist, or 

 to a patch of sand suitable for burrowing, it begins to bury itself 

 in a manner similar to that of M. ventricosa, only much more rapidly. 

 It moves sand away from its ventral surface (pi. 8, figs. 1-3) and up 

 along its sides into two crescentic mounds. When its dorsal surface 

 is lowered nearly to the surface of the sand, it brings sand up onto 

 that surface (pi. 8, figs. 4, 5), thus becoming effectively covered be- 

 fore the two lateral crescents of sand actually coalesce over the test. 

 It takes only about 3 minutes to bury to a depth of about half the 

 thickness of the test, and another 5 minutes to descend nearly to the 

 level of the surface of the sand. Complete burial can be accomplished 

 in 10 minutes or slightly more or less, depending upon the individual. 

 The animal does not attempt to move forward until burial is complete 

 or nearly so. 



Predation. — Moore (1956) reports that P. grandis is frequently 

 preyed upon by species of the helmet conch, Cassis. Many of the dead 

 tests in our collection show the small circular hole that is the mark 

 of predation by a gastropod. Dead tests and living specimens were 

 most abundant at station 30, and there two specimens of Cassis were 

 observed, a large one about 12 inches long, and a smaller one 4 inches 

 long that was buried with its dorsal surface about 4 inches below the 

 surface of the sand. These associations led us to suspect that Cassis 



