250 THE BIOLOGY OF THE SEA-SHORE 



of at once taking shelter under stones, shells, etc., and of 

 fending for themselves. 



As has been already noted, shore Crustacea retire to 

 deeper water to spawn, probably because the temperature 

 becomes more stable with increasing depth. 



The tidal rhythm has in several cases had a profound 

 effect on the timing of the life-cycle. This is well seen in 

 Convoluta roscqffensis, the tiny ciliated Planarian worm con- 

 taining green symbiotic algae which we have mentioned as 

 living in large colonies on the sandy shores of Normandy 

 and Brittany, just at the level reached by high water at 

 the slackest of neap tides. We have already seen (p. 239) 

 that with the onset of the tide the patches disappear 

 as the animals retreat beneath the surface till the next 

 ebb-tide. Twice each day, for a period of six hours, the 

 animals live underground, and twice they rise to the surface 

 and remain there for a similar period. These vertical 

 movements are imposed on the animal, the burrowing 

 reaction being due to wave-shock and the upward move- 

 ment to the influence of light in relation to the symbiotic 

 algae. Now, it so happens that at Roscoff in the summer the 

 low water of spring tides falls at midday and midnight, and 

 consequently when the Convoluta zone is uncovered for 

 the second time, the animals in the absence of light are not 

 obliged to come to the surface. It is precisely at this period, 

 when the worms are able to remain below the sand for the 

 longest possible time, viz. eighteen hours at a stretch, that 

 egg-laying, as observed at Roscoff^, reaches its maximum 

 (Keeble, 1910). 



Perhaps a still more striking example of a similar pheno- 

 menon is afforded by the " Grunion " {Leuresthenes tenuis), 

 a small smelt about 5! to 6| inches long occurring on the 

 sandy shores of California. On moonlight nights during the 

 big tides of March, April, May, and June — on the 2nd, 3rd, 

 and 4th nights after full moon, very shortly after high tide, 

 to be exact — the Grunion comes in with the sweep of the 

 water and, as the waves break, lies for a moment, squirms 

 and drops back into the wash of the next wave. Crowds of 



