No. 624] SHORTER ARTICLES AND DISCUSSION 



ON EE GENERATION AND THE RE-FORMATION OF 

 LUNULES IN MELLITAi 



The ambulacral lunules of the genus Mellita are characteris- 

 tically developed by the inclusion of reentrant marginal notches ; 

 except that in M. sexiesperforata these lunules, like the inter- 

 ambulacral lunule in this and in related genera, is formed by 

 resorption through the test.^ In a few species the ambulacral 

 lunules are permanently maintained as open marginal notches, 

 and Jackson^ has called attention to the fact that in Encope 

 annectans Jackson, the adult interambulacral lunule is repre- 

 sented by mere dorsal and ventral furrows, not sufficiently deep 

 to meet and form a hole through the test. In view of the possibly 

 exceptional character of the formation of the ambulacral lunules 

 in M. sexiesperforata, note may be made of the manner in which 

 the lunules of this species are re-formed during regenerative 

 changes consequent upon natural injury. For in collections of 

 Mellita comprising half-a-dozen or more individuals of adult 

 size it is rarely found that every specimen presents a complete 

 and regular marginal outline. The degree of irregularity is in 

 most instances not large, but in some cases amounts, at the 

 deepest point, to a radial deficiency of 13 mm. in a specimen 12 



this character in specimens less than 8 cm. in t laMsvrt-.' diain- 

 eter. The nature of these deficiencies is such a> tn that 

 they have been inflicted by other bottom- feeding animals, pos- 

 sibly fishes. The wound-surfaces seem, however, to be readily 

 covered over; and the various degrees of alteration toward a 

 more perfect outline, exhibited in different specimens, show that 

 relatively complete restitution is possible. 



It is a noteworthy fact that these injuries are almost entirely 

 confined to that end of the animal which is morphologically the 

 posterior (cf. Figs. 1, 2, 3). It seems to me possible that this 

 fact may be understood upon the assumption that when burrow- 

 ing the anatomically anterior end of the "sea plate." which is 

 somewhat more sharply pointed than the posterior, is the one 



1 Contributions from the Bermuda Biological Station for Rt^<>ar.h. 



2 Agassiz, A., "Revision of the Echini," pp. 320-324. Clark, H. L., 1904, 

 "Echinoderms of the Woods Hole Region," Bull. U. S. Bur. Fish, for 1902, 

 pp. 545-576 (p. 565). 



3 Jackson, R. T., 1917, Proc. U. S. Nat. Mus., Vol. 53, pp. 489-501 (p. 494). 



