BEHAVIOUR OF SHORE ANIMALS 239 



we may cite that of the sea-cucumber Cucumaria cucwnis. 

 When placed upon a vertical plate of glass or slate, the animal 

 creeps upward until it reaches the highest level, v^here it 

 remains. It is sufficient now to turn the glass through 

 180° round an horizontal axis to induce the animal to move 

 upwards once more, and this behaviour continues indefinitely 

 as long as the glass is rotated. It has been shown that this 

 behaviour takes place independently of any influences of light 

 and oxygen (Loeb, op. cit.). 



Rhythmic Behaviour on the Shore. — The rhythmic 

 nature of many biological phenomena has long been observed, 

 and in analysing behaviour the effect of these periodicities 

 must not be ignored (see Flattely, 1920). They play a 

 particularly important part on the shore. The majority 

 of shore animals are, as Bohn points out, subjected to a 

 double periodic influence, the rhythmic ebb and flow of the 

 sea and the alternation of day and night, to which they are 

 constantly reacting. This is well seen in the case of Con- 

 voluta, a tiny ciliated flat-worm which occurs on flat sandy 

 shores, such as those of Normandy and Brittany. 



The much discussed form, Convoluta roscqffensis, is 

 green in colour owing to the presence of unicellular green 

 plants living with it symbiotically. It occurs in immense 

 colonies which appear, at low tide, as great patches of green 

 scum about halfway down the shore. As the tide laps the 

 edges of the colony the green patches disappear, the worms 

 retreating below the surface and remaining there till the next 

 ebb-tide. Twice during the twenty-four hours the zone 

 occupied by the colonies is submerged and the animals live 

 in darkness underground, and twice the zone is uncovered 

 and the animals rise to the surface. The burrowing reaction 

 is due to the necessity of avoiding extermination by wave- 

 shock, the upward movement is determined by the presence 

 of the algal cells and their light requirements. The worms 

 thus show up-and-down movements synchronous with 

 those of the tides, but in an inverse direction (see Keeble, 

 1910). 



This constant reaction to the tidal rhythm has had a 



