n] RHYTHMICAL CHANGE IN THE SEA 39 



spring tide : the eggs are laid then because for a 

 longer period than usual the sands are covered with 

 water owing to the higher rise of the tide. But we 

 find that this tidal rhythm is very deeply impressed 

 on the conscious or unconscious memory of the 

 animals. If we attempt to keep cockles in an aquarium 

 for a long time it becomes difficult to keep the animals 

 in good health, and we can improve the conditions by 

 running away the water of the tank for a time every 

 day in order to imitate the action of the tides. If we 

 keep Convoluta in vessels of water in the laboratory 

 under uniform conditions they will still burrow 

 beneath the surface of the sand at the time when the 

 tide is ebbing from off the beach, although the vessel 

 is kept continually full of water. There are similar 

 rhythms of habits in many other animals. When the 

 sea is phosphorescent in some parts of our waters the 

 luminosity is usually due to a protozoan called 

 Noctiluca, or to copepods, or to some peridinians. 

 All these animals become phosphorescent only when 

 it is dark, that is after sunset. But if we capture 

 them and keep them in a photographer's dark room 

 we find that they become phosphorescent there only 

 when it is dark outside under natural conditions: 

 that is the animals seem to remember the regular 

 alternation of day and night, even when they are kept 

 under strictly uniform conditions. The Chameleon 

 Shrimp, Hippolyte, shows a similar rhythmic habit. 



