KOFOID AND SWEZY: UNARMOKED DINOFLAGELLATA 51 



Beyond the application of stimuli for the production of light by the organisms, 

 no laljoratory experiments with them have been carried on by us. In their 

 reaction to light a few facts only have been brought forth. 



One of the most interesting of these is that recorded by Herdman (1913) 

 for two species of Amphidiniam. He had noted, during a succession of years, 

 the rhythmic appearance of patches of these organisms on the beach sands at 

 Port Erin, Isle of Man. In addition to this rhythmic occurrence another 

 rhythm was observed in the appearance above, and disappearance below, the 

 surface of the sand ; it was these latter movements that were investigated. It 

 was found that this was a direct response to environmental stimuli and could 

 be very fairly calculated from a knowledge of the actinic value of the light 

 and the condition of the tide. The optimum light was a diffused one, with an 

 avoidance of strong illumination. The organisms were also found to disappear 

 before the incoming tide reached them and to reappear as soon as the tide re- 

 ceded from their area. These two interacting stimuli, light and tide, produced 

 a daily and a fortnightly rh\i;lnn. If conditions were experimentally altered 

 a new rhythm intervened. 



During the summer at La Jolla hauls were made at four-hour intervals 

 through the day aud night from the surface of the water at the end of the pier 

 at the Biological Station. These were supplemented by hauls made at varying 

 distances of one to eleven miles offshore, both surface hauls and from a depth 

 of sixty or eighty meters to the surface. The latter were made only during 

 the day, usually about 8 o 'clock in the morning, though this was not invariable. 

 These haiils were carefully examined and records made of the occurrence of 

 the dinoflagellates contained therein. 



These records during the month of July for the collections at the pier show 

 only a slight variation in the number of species present at the different intervals 

 throughout the day, the smallest number being that for 4 a.m., the highest at 

 8 a.m., with a slightly smaller percentage at 8 p.m. and 12 ji.m. than for the 12 m. 

 and 4 p.m. records. These records are inconclusive owing to the amoimt of 

 foggy weather prevailing during that month. This was foi;nd to influence 

 materially the nmnljer of holozoic dinoflagellates (or those without chromato- 

 phores) appearing at the surface. Most of the forms possessing chromato- 

 phores, and therefore dependent upon sunlight, are found to be neritic or littoral 

 in their habitat, with a predominance of holozoic organisms in the strictly 

 pelagic plankton. As a result of this, surface hauls made during the day 

 contain the neritic foi'ms and relatively few holozoic species. 



In the hauls made farther offshore, where neritic influences were not so 

 potent, it was found that the surface hauls usually contained relatively few 

 foi-ms, both as to species and as to individuals, while the deeper hauls made at 

 or very near the same phv^e at approximately the same time were usually well 

 supplied with dinoflagellates. 



These conditions indicate a negative heliotropism, with marked avoidance 

 of strong sunlight in many of the species at least. 



