16 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



continued application of the stimuli and are not " habits," as in 

 the case of Convoluta ; for when a bucketful of sand, containing 

 the organisms, is brought into the laboratory, they remain at 

 the surface, whatever the state of the tide may be, unless 

 covered by water. There is also a lunar rhythm, as pointed 

 out by Laurie, periods of great abundance and activity at 

 spring tides, alternating with periods of scarcity at neaps. 

 Laurie accounts for this on the grounds that, at Hoylake, 

 where most of his observations were made, high water at 

 neaps does not cover the region occupied by Amphidinium. 

 From my observations at Port Erin, I am inclined to attribute 

 this rhythm rather to the same causes as the diurnal rhythm, 

 viz., light and tide. Low water of springs on this part of the 

 coast occurs in the morning and evening, and hence the rhythm 

 caused by the stimulus of light will coincide with that caused 

 by the stimulus of tide, giving rise to a greater amplitude of 

 variation than is possible at neaps. Both explanations are 

 probably true to some extent. The third rhythm noted by 

 Laurie is a seasonal one. He records a maximum from 

 February to April, after which the discolouration of the sand 

 decreases in intensity and extent, and is barely distinguishable 

 in the late summer. This discolouration was produced by the 

 " short form " of A. operculatum since placed by Kofoid in a 

 new species, A. herdmani. Laurie also records a small patch 

 of the " long form," A. asymmetricum, which was first noticed 

 on July 6th, and which persisted through the late summer. 

 Kecent observations at Port Erin confirm these results. 

 A. herdmani is the principal cause of the discolouration in 

 spring, while later, especially during the hot summer of 1921, 

 it is almost absent and is replaced to a large extent by 

 A. asymmetricum and other forms not observed by Laurie. 



During the summer of 1920, efforts were made, but 

 without success, to observe the division stages of Amphidinium. 

 Although, both on the shore and in the laboratory, these 



