HOLOTHURIANS
Heretofore the only paleontologic evidence of the Holothurians has been the presence, in rocks of late Paleozoic and post Paleozoic age, of the spicules of those forms having a calcareous subepidermic skeleton. To find, in the Middle Cambrian, representatives of the Actinopoda, both with and without podia, and a form indicating a second order, Paractinopoda, is a great surprise. This establishes
 Fig. 2.—Diagrammatic reconstruction of the imagined primitive Pelmatozoic ancestor. (After Lankester, 1900, fig. 7, p. 9[1]) O = mouth; As = anus; ac = right and left anterior portion of cœlom; rpc and lpc = right and left posterior portion of cœlom; lhc = left hydrocœl; sc = canal connecting lhc and ac; par = parietal canal; M = dorsal pore; pl = preoral lobe with nerve center n.  Fig. 3.—Diagrammatic reconstruction of imagined Dipleurula ancestor. Anterior end at left of drawing; organs of left side toward observer, and with stronger outline than those of right side. (After Lankester, 1900, fig. 1, p. 4.[1]) O = mouth; As = anus; ac = right and left anterior portion of cœlom; rpc and lpc = right and left posterior portion of cœlom; rhc and lhc = right and left hydrocœls; sc = canal connecting lhc and ac; M = dorsal pore; pl = preoral lobe with nerve center n. |
the very ancient origin of the Class Holothurioidea and the fact of its great differentiation in Middle Cambrian time. This is particularly true of the free swimming, pelagic form, Eldonia ludwigi.
Among zoologists the theoretically most primitive ancestor of the
