236 THE BIOLOGY OF THE SEA-SHORE 



show a tendency to bring their bodies completely into contact 

 with solid objects, and this is particularly marked in many 

 shore creatures. One of the commonest shore Polychasts, 

 Cirratulus tentaculatus , always occurs beneath flat stones 

 embedded in sandy mud, as much of the body as possible 

 being kept in contact with the under side of the stone. When 

 transferred to a dish of sea- water where there are no stones, 

 the worm roams about restlessly. If a flat stone is now 

 introduced into the dish the animal will be found, in a very 

 short time, firmly ensconced beneath it and looking distinctly 

 more comfortable (Flattely, 1916). Maxwell (quoted by 

 Loeb, op. cit.) placed a number of specimens of Nereis in a 

 porcelain dish devoid of sand, but containing glass tubes 

 of varying sizes to correspond with the different sizes of the 

 worms. After twenty-four hours every tube was inhabited 

 by a worm which had made it its permanent abode, not 

 stirring even when exposed to sunlight although this caused 

 its death. This attraction or repulsion by solid surfaces is 

 known as stereotropism or stereotaxis and is a phenomenon 

 of great importance. 



Negative stereotaxis is seen, according to Loeb, in the 

 larvae of the barnacle at a certain stage and of other Crus- 

 taceans. 



Different parts of the same organism may show different 

 responses. It is in this way that Loeb accounts for the 

 righting reactions of starfishes, etc., their ventral surface 

 being positively phototropic, and their dorsal surface nega- 

 tively so. That this phenomenon is quite distinct from 

 geotropism is demonstrated by Loeb's observation that 

 starfish will cling to solid surfaces with their tube feet, 

 even if by doing so their dorsal surface is turned towards the 

 ground. 



Similarly, the stolons of hydroids will cling to solid 

 bodies, while their polyps bend and grow away at right 

 angles. The regenerating polyp of Tubularia mesembry- 

 anthemum, the stem of which is in contact with the glass 

 side of an aquarium, will grow out at right angles to the 

 vertical wall (Loeb, op. cit.). 



