IX] 



OF HOLOTHURIAN SPICULES 



687 



nema to acquire their reflexed spokes, but the perforated plate is 

 more comprehensible. Each plate starts in a httle clump of cells 

 in whose boundary-walls calcareous matter is deposited, doubtless 

 by adsorption, the holes in the finished plate thus corresponding to 

 the cells which formed it. Close-packing leads to an arrangement 

 of six cells round a central one, and the normal pattern of the plate 

 displays this hexagonal configuration. The calcareous plate begins 

 as a little rod whose ends fork, and then fork again: in the same 

 inevitable trinodal pattern which includes the "polar furrow" of 

 the embryologists. The anchor had been first formed, and the 



Rh 



r^^O 



Fig, 315. Various holothurian spicules. After Theel. 



little plate is added on beneath it. The first spicular rudiment of 

 the plate may lie parallel to the stock of the anchor or it may lie 

 athwart* it. From the physical point of view it would seem to be 

 a mere matter of chance 'which way the cluster of cells happens to 

 he; but this difference of direction will cause a certain difference 

 in the symmetry of the resulting plate. It is this very difference 

 which systematic zoologists at one time seized upon to distinguish 

 S. Buskii from our two commoner "species." The two latter 



* Cf. S. Becher, Nicht-funktionelle Korrelation in der Bildung selbstandiger 

 Skeletelemente, Zool. Jahrbucher (Physiol.), xxxi, pp. 1-189, 1912; Hedwiga 

 Wilhelmi, Skeletbildung der fiisslosen Holothurien, ibid, xxxvii, pp. 493-547, 

 1920; Arch. f. Entw. Mech. xlvi, pp. 210-258, 1920. See also W. Woodland, 

 Studies in spicule-formation, Q.J. M.S. xlix, pp. 535-559, 1906; li, pp. 483-509, 

 1907 and R. Semon, Naturgeschichte der Synaptiden, Mitth. Zool. St. Neapel, vii, 

 pp. 272-299, 1886. On the common species of Synapta, see Koehler, Faune de 

 France, Echinodermes, 1921, pp. 188-9. 



