176 THE BIOLOGY OF THE SEA-SHORE 



withdraw to the very lowest points of their burrows. Tubi- 

 colous Polychaets are similarly retracted within their tubes. 

 The only evidence of the life below is afforded by worm- 

 casts or by small holes and depressions in the sand. Very 

 different is the appearance of the same stretch of shore 

 when covered with water. The burrowing worms are now 

 swimming or moving about freely. Tube-dwelling forms 

 have climbed up inside their tubes and protruded a crown 

 of tentacles which are waving in the water. A similar 

 vertical ascent has been made by bivalves, the siphons of 

 which now project above the surface of the sand. 



Williamson (1907) has noticed that when the water 

 was let out of a box to the sides of which a number of 

 small mussels were attached, the animals slackened their 

 byssi in order to keep themselves in the water. When, 

 however, having reached the end of their tether, the water 

 was still receding, they hauled themselves up tight against 

 the wall of the box again. This power of adjustment, 

 though slight, might just make the difference between 

 annihilation and safety, and seems to be adaptive. 



Homing. — The fact that the limpet, with the edge of 

 its shell, forms an impression or ** scar " on a rock on which 

 it has been living (if this is not too hard) long ago drew 

 attention to the movements of this animal. The outline 

 of the scar exactly corresponds to that of the shell-margin 

 (by the mechanical action of which it is formed), so that if 

 a limpet moves it must not only return to the identical spot 

 or " home," but must orient itself exactly as before. It has 

 now been definitively established that when once a limpet 

 has taken up a fixed position on a rock, it only leaves it to 

 make journeys for food and always returns to the same 

 spot, though there seems to be a certain amount of difference 

 of opinion as to when the movements are effected. 



According to Ainsworth Davis (1895), the " homing " 

 instinct of the limpet is well developed, and in this writer's 

 belief the " locality sense " is independent of smell, sight, 

 and touch, at least as far as the head tentacles are concerned. 

 The greatest distance from which a limpet has been known 



