Page:Walcott Cambrian Geology and Paleontology II.djvu/84

 Echinodermata is considered to have passed through a "Pelmatozoic" stage in which the animal was attached to some object by a part of its body wall, and in which the mouth and, to a less extent, the other apertures faced upward. This stage is represented by text fig. 2.

Selecting the characters common to the early stages of all Echinoderms, a diagrammatic reconstruction of this imaginary phylogenetic stage gives a marine animal with the longer antero-posterior axis parallel to the sea floor. The mouth was antero-ventral, anus posterior or postero-ventral, the two joined by an uncoiled gut with perhaps a stomachal enlargement in the middle as represented by fig. 3.

The simplest larval form among recent echinoderms, Auricularia of the Holothurians, differs from fig. 3 in being bent upon its ventral surface so that the mouth lies in the middle of a concavity and the anus on the ventral surface of the lobe back of the concavity. It also shows a decided change in the arrangement of the coil of the alimentary canal and the cœlomic cavities, as may be seen by comparing figs. 3 and 4.

I have mentioned the theoretical ancestor of the echinoderm and of one of its classes, Holothurioidea, in order to note that the zoologist has not carried his theoretical line back to the period when the ancestral form was pelagic and had not yet adjusted itself to the conditions of the littoral zone, stages which must have preceded the migration of this organism over the bottom into the deeper water. This still earlier ancestor must have been a free swimming, soft bodied animal. It undoubtedly was more simple than the free swimming Eldonia ludwigi described in this paper, and I can readily imagine a small bell-shaped body with a simple alimentary canal opening at both ends on the ventral surface—a medusa-like object