No. 624] SHORTER ARTICLES AND DISCUSSION 



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which is carried ahead.* These animals do burrow beneath the 

 surface of the sand when the weather is at all stormy, and, if 

 this assumption be valid, the posterior end might then be exposed 

 (or even separated somewhat from the bottom) to a greater 

 extent than at other times. If the process of emergence from the 

 sand is somewhat different from, or quicker than, that involved 

 in burrowing, one could understand why the anterior end is 

 rarely, if ever, damaged, as might otherwise be expected if dif- 

 ferential exposure of some kind alone determines the incidence 

 of injuries ; there are, of course, other possibilities. 



However they originate, the restorative phenomena which these 

 posterior injuries entail show that the ambulacral lunules of 

 M. sexiesperforata May, in regeneration, follow a method of for- 

 mation resembling, in a measure, that adhered to in the normal 

 development of these lunules by other mellitas. The individuals 

 herewith depicted in outline (Figs. 1, 2, 3) exhibit several stages 

 in a process of lunule-completion through the concrescence of 

 the growing edges of the disc. It is difficult to decide whether 

 this process is of a specific regulatory character, "aiming at" the 

 reconstitution of the lunules, or whether it represents merely the 

 inevitable consequence of ordinary (though accelerated) growth 

 at the margin of the mellita disc, and is, perhaps, for this reason, 

 devoid of any recapitulatory significance. An inspection of 

 Fig. 1 will show that at a there is evident a decided out-bulging 

 of the disk-margin, at the point of union with the old outline of 

 the lunule. This out-bulging, seen also at /? in Fig. 2, and at /8 

 in Fig. 3, shows definite growth of the tissue toward the opposite 

 lunule-wall in interambulacrum V. At p in Fig. 1 an outgrowth 

 of this type has met and fused with a less extensive outgrowth 

 from the opposite lunule-boundary ; here, as at a in Fig. 2, it will 

 be noted that the lateral extension of interambulacral area V is 

 not confined merely to the margin of the disc, but affects also the 

 whole lateral wall of the lunule on that side,— provided the injury 

 be sufficiently extensive — so that closure of the lunule is slow. 

 If the original disturbance be small, as at /3 in Fig. 1, this and 

 other similar cases show that reparation may be relatively com- 

 plete. On the other hand, more extensive injurj-, as at in Fig. 

 3, seems to result in "regeneration" which is not so quickly 

 effective as, for example, in /?, Fig. 1 ; under these circumstances 

 the posterior extension of the substance of interambulacrum I, 

 * Cf. Cole, L. J., 1913, Jour. Exp. Zool., Vol. 14, pp. 1-32. 



