Anchors of Holothurians 31 
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 improve- 
ments, 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 short, every little adaptation in the modelling of the 
anchor must have possessed selection-value. And that such minute 
changes of form fall within the sphere of fluctuating variations, that 
is to say, that they occur is beyond all doubt. 
In many of the Synaptidae the anchors are replaced by 
calcareous rods bent in the form of an 8, which are said to 
act in the same way. Others, such as those of the genus 
Ankyroderma, have anchors which project considerably beyond the 
skin, and, according to Oestergren, serve “to catch plant-particles 
and other substances” and so mask the animal. Thus we see that 
in the Synaptidae the thick and irregular calcareous bodies of the 
