Page:Walcott Cambrian Geology and Paleontology II.djvu/83



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

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