7. Anthozoa (incl. Hydrocorallia). A. Zoanthana. 23 



Bohnf 1 ) has compared the rhythm exhibited by Actinia equina at Yport, Nor- 

 mandy, and Banyuls. At Yport the water in the tidal pools varies in oxygen 

 content, etc. from day to day, and thus the nyctheraeral rhythm is dis- 

 turbed and the reaction of the anemones depending on other conditions, e.g. 

 in well lighted pools, saturated with oxygen, the anemones remain closed while 

 in shaded pools, poor in oxygen, they expand. When these perturbing factors, 

 produced by the tides, are eliminated, all the anemones present a very marked 

 nycthemeral rhythm, as is clearly observed at Banyuls. There the anemones 

 live in shallow water the temperature and chemical composition of which 

 undergo slow periodic variations, which are small compared to those of light 

 (day and night). The anemones remain closed or scarcely half open during 

 the day but expand fully from twilight, that is, when the light factor only 

 has varied; they close at dawn. This rhythm persisted for a few days when 

 the anemones were kept in continuous darkness. In spite of the acquired 

 periodicity, if at night a lighted candle is brought near them, they close at 

 once. The nycthemeral rhythm of actiniae is determined undoubtedly by the 

 factor of light; chemical variations in the water only perturb it. 



Bohn( 2 ) observed the behaviour of anemones on the south coast of Brittany 

 at low temperatures, and found that the later in the morning low tide occurs 

 the more rapid is the closure of the anemones; on cloudy or foggy days, when 

 the light is feebler, the anemones close more slowly. At night, in spite of 

 low tide, the anemones expand considerably, sometimes they are not closed 

 even when completely out of the water. These observations indicate that the 

 closure of actinia at low tide is determined only by increased illumination. 

 - Bohn( 3 ) states that in shore pools, at low tide, Actinia equina, closed under 

 the influence of light, may re-expand at the end of a certain time, this being 

 shorter when the light is more intense and the temperature high. The re- 

 expansion is a consequence of insensibility to light: when it takes place the 

 anemone has ceased to react to variations in illumination and, in general, its 

 sensibility to various external stimuli is much decreased. In full daylight the 

 animal is henceforth insensible to light and behaves as in the night, that is, it 

 expands. Anemones belonging to various species close in the morning under 

 the influence of light but expand during the day and, when night comes on, remain 

 expanded, having become insensible to the change of illumination. This diurnal 

 expansion, due to insensibility, takes place in water containing various amounts 

 of oxygen and of various temperatures. Rhythmic phenomena and their 

 perturbation cannot be explained only by reference to the factors in operation 

 at the time of observation, for the same individual may behave differently on 

 two successive days according as its chemical activity has been great or feeble 

 during the preceding hours, i.e. its state of insensibility more or less pro- 

 nounced. 



Bohn( 4 ) transferred Actinia equina, which at low tide had been subject to 

 considerable dessication, to an aquarium and found that alternate expansion 

 and contraction took place, corresponding to the periods of high and low water, 

 especially if slight shocks were given from time to time. After 3 or 4 days 

 the rhythm become feebler and at the end of 8 days the existing influences 

 had overcome the rhythm. There appear to be two physiologically different 

 races of Tealia crassicornis, corresponding to two rhythms, namely, those which, 

 in their natural habitat, were protected from strong light and, in aquarium, 

 remain closed in strong light, opening at night, and those which lived in well 

 lighted pools, which expand fully during the day and close at night. Similarly 

 A. e. from near high tide mark present, in aquarium, a periodicity correspond- 



Zool. Jahresbericht. 1910. Coelenterata. <r 



