AMPHIDINIUM OPEECULATUM AT PORT ERIN. 73 



Irish Sea during the last four years, and which have been examined in minute 

 detail by Mr. Andrew Scott, A.L.S. 



Thinking it might be present in the shallow water close to the edge o£ the 

 beach, Mr. W. Riddell and I took some hauls of the tow-net from a punt 

 worked backwards and forwards in a few inches of water as near as we could 

 get to the discoloured sand, but the gathering, although it contained fine 

 sand and mud, showed no trace of our Dinoflagellate. It may be noted here 

 that although the size of the Amphidinmm, 0'05 mm. in greatest diameter, is 

 such that it can slip through the mesh (averaging about 0*08 mm.) of the 

 finest plankton silk (no. 20), still so much clogging of the meshes always 

 takes place in such hauls, and so many other smaller organisms and particles 

 of mud are retained, that it is certain that had the AmpJiidinimn been 

 present in any quantity in the water it would have shown up in ihe 

 gatherings. 



Careful scraping of the sand showed that the Dinoflagellates were only in 

 and on the surface-layer, a-nd therefore could not be regarded as coming up 

 from below. It occurred to us that possibly they might be fresh- or brackish- 

 water forms derived from the land ; but we ascertained that the little 

 stream in the centre of the bay, which in wet weather overflows on to the 

 beach (at other times it is conveyed into the town sewer), had not, on account 

 of the unusually dry season, sent any water to the beach for some weeks. 

 Moreover, on experimenting with the living AmpJiidininm in the laboratory, 

 we found that while it lived well in sea-water it died at once in fresh and 

 survived for a few days only in brackish water. Samples of the AmpU- 

 dinium kept in shallow dishes of wet sand at the Biological Station in a few 

 days showed such profuse growth that the sand was covered by a dark- 

 coloured layer, the water became impure, and eventually all the Dinoflao-ellates 

 died off. 



Observation under the microscope shows that although this is a singularly 

 active Dinoflagellate, circling round and round with great vigour, so that a 

 drop of sand and water containing a number of the organisms presents a most 

 animated picture under a low power magnification, still the Amplddinimn 

 seems to be actually attracted to the sand-grains and associated with them. 

 The sand-grains in the field of view are always peppered over with a number 

 of specimens of the Ampludinkua (PI. 8. figs. 2, 3, 4), and if individuals be 

 watched they are seen after swimming round to come back to rest on a 

 sand-grain and remain there for a time before starting off on another 

 excursion. If they are thus constantly associated with sand-grains or other 

 sohd particles, and never swim more than a microscopic distance from such 

 a resting-place, that may account for the fact that we have never found them 

 in our plankton gatherings. 



Ampliidinium operculatum is also, however, positively heliotropic, cono-re- 

 gating in quantity on the lighter side of the dish in the laboratory, and 



