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THE AMERICAN NATURALIST [Vol. LIV 



point may shift progressively from side to side during an ex- 

 tended act of creeping, but at all times some part within the two 

 anterior interradii is in advance. Mellita can, however, pivot 

 in complete circles, in either direction, about its mouth as a 

 center; it also carries out successive incomplete swings, alter- 

 nately opposite in direction. Movements of the latter type, 

 together with the relatively fixed direction of creeping, namely 

 anteriorly, are important for the act of burrowing, and are cor- 

 related with some notable growth-changes in the form of the 

 whole body. 



During burrowing the anterior end is in advance. The process 

 of concealment is a fairly rapid one, a large specimen being able 

 to disappear completely in less than 15 minutes, although two 

 to three times this interval may be employed. Not only do the 

 spines and tube feet assist in clearing a way through the sand, 

 by moving the individual sand grains, but, in addition, the 

 body as a whole is used as a digging instrument. The disk is 

 repeatedly rotated 30° or more to either side of the sagittal line, 



with the result that, since the very numerous spines and tube 

 feet are simultaneously pressing the disk forward, the animal is 

 actually insinuated, or "slid," into the ground. This maneuver 

 is especially effective upon a muddy bottom, where movement of 

 sand by the tube feet and spines would be a slow and inefficient 



The young Mellita is quite thin and wafer-like, its outline 

 practically circular (Fig. 1). As the animal grows, the thick- 

 ness of the body increases. although the edge of the disk remains 

 thin. In many cases the outline of the disk is still almost cir- 



