ound 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 enabled by means of them to bore rapidly into the ground and so to
escape from enemies. We do not know what the initial stages were, but we
cannot doubt that the little improvements, which occurred as variations
of the originally simple slimy bodies of the Holothurians, were
preserved because they already possessed selection-value for the
Synaptidae. For such minute microscopic structures whose form is so
delicately adapted to the role they have to play in the life of the
animal, cannot have arisen suddenly and as a whole, and every new
variation of the anchor, that is, in the direction of the development
of the two arms, and every curving of the shaft which prevented the tips
from projecting at the wrong time, in sh