creeping, they press firmly with their tips, which are embedded in the
skin, against the substratum on which the animal creeps, and thus prevent
slipping backwards. In other Holothurians this slipping is made impossible
by the fixing of the tube-feet. The anchors act automatically, sinking
their tips towards the ground when the corresponding part of the body
thickens, and returning to the original position at an angle of 45 degrees
to the upper surface when the part becomes thin again. The arms of the
anchor do not lie in the same plane as the shaft, and thus the curve of the
arms forms the outermost part of the anchor, and offers no further
resistance to the gliding of the animal. Every detail of the anchor, the
curved portion, the little teeth at the head, the arms, etc., can be
interpreted in the most beautiful way, above all the form of the anchor
itself, for the two arms prevent it from swaying round to the side. The
position of the anchors, too, is definite and significant; they lie
obliquely to the longitudinal axis of the animal, and therefore they act
alike whether the animal is creeping backwards or forwards. Moreover, the
tips would pierce through the skin if the anchors lay in the longitudinal
direction. Synapta burrows in the sand; it first pushes in the thin
anterior end, and thickens this again, thus enlarging the hole, then the
anterior tentacles displace more sand, the body is worked in a little
farther, and the process begins anew. In the first act the anchors are
passive, but they begin to take an active share in the forward movement
when the body is contracted again. Frequently the animal retains only the
posterior end buried in the sand, and then the anchors keep it in position,
and make rapid withdrawal possible.

Thus we have in these apparently random forms of the calcareous bodies,
complex adaptations in which every little detail as to direction, curve,
and pointing is exactly determined. That they have selection-value in
their present perfected form is beyond all doubt, since the animals are