Anchors of Holothurians 31 



the curve of the amis 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 Avay, above all 

 the fomi 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 contmcted 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 adai)ted to the 

 rule tliey 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 |)ossesscd selection-value. And that such minute 

 changes of form fall within the sphere of fluctuating variations, that 

 is to say, tltnt thef/ occur is beyond all doubt. 



In many of the Synaptidae the ancliors 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 

 Ankyrodcrma, have anchors which project considerably beyond the 

 skin, and, according to Oestergren, serve "to catcli 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 



