422 SCIENCE PROGRESS 



beneath the surface till the next ebb-tide. Twice during 

 24 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 burrow- 

 ing reaction is due to the necessit}' of avoiding extermination 

 by wave-shock, the upward movement is determined by the 

 presence of the algal cells and their light requirements. The 

 egg-laying of Convoluta roscoffensis is also periodic, and is 

 related in a remarkable way to the rhythm of the tides. Egg- 

 laying begins with the onset of the spring tides and continues 

 for a week. The reason for this is as follows : In the summer, 

 the time of year when these observations were made, the 

 low-water of spring tides at Roscoff occurs about midda}^ and 

 midnight. 1 When, however, the zone occupied by the Convo- 

 lutas is uncovered during the night-time, the animals in the 

 absence of light do not rise to the surface. Hence, during 

 the spring tides, the worm has an uninterrupted period of 

 some 18 hours in which to lay its eggs. Experiments in the 

 laboratory have shown that egg-laying reaches its maximum 

 when the animals are not obliged to come to the surface twice 

 in 24 hours — that is to say, when they can have the longest 

 possible spell of darkness ; in other words, the conditions 

 most favourable to egg-laying occur when the moon is full or 

 new — a remarkable example of the effect of the tides on the 

 habits of shore animals. 



Shore animals like the common periwinkle, for instance, 

 are submitted to a double periodic influence : the rhythmic 

 ebb and flow of the sea and the alternation of day and 

 night. The existence of periwinkles comprises regularly alter- 

 nating periods of active life in the water or moist air (at high 

 tides) and periods of suspended animation within their shells. 

 This constant reaction to the tidal rhythm is not without a 

 profound influence on the functions of the organism. For in- 

 stance, inert periwinkles, even in a dry environment, can be 

 reactivated by shaking ; but, according to Bohn, the reactiva- 

 tion occurs much more readily at certain times and hours. 

 Bohn 2 states that if a collection of these molluscs has been 

 isolated for a certain length of time in a laboratory, it is easily 

 demonstrated that, at periods of low tide, one has to shake 

 much longer to produce the reactivation than when the tide 

 is high. That is to say, the periods of inertia in the laboratory 

 correspond to the periods of desiccation on the shore. 



The impress of the external rhythm on the organism is, 



1 Keeble, F., Plant-Animals. (Cambridge : University Press, 1910.) , 



2 Bohn, Georges, Institut Psychologique : Travail du Groufie d 1 Etude de 

 Psychologie zoologiquc. Mcmoire I : Attractions et Oscillations des Animaux 

 viarins sous I'influence de la lumihre. (Paris, 1905.) 



