ON THE ARTIFICIAL CULTURE OF MAltlNE PLANKTON ORGANISMS. 457 



Salinity, apart from the quantities of available nutrient materials, 

 can be varied within large limits without appreciable effect on the 

 diatoms. 



II. MIXED CULTUEES. 



In what has been said up to the present, we have been dealing with 

 persistent cultures containing a single species of diatom, which are 

 comparatively, if not entirely, free from admixture of other organisms. 

 Tlie study of cultures which contain a considerable mixture of organ- 

 isms is not without interest. 



A number of experiments have been made on the following lines. 

 About 10,000 cc. of water, taken at some distance from shore, was 

 placed in a tall bell-jar fitted with a " plunger," which keeps the water 

 in constant movement. (Journ. Mar. Biol. Assoc, Vol. 5, p. 176). 

 The water was treated with Miquel's solutions in normal propor- 

 tions, and a considerable quantity of plankton taken with a fine-meshed 

 net (150 meshes to the inch) was added, say, 10 or 20 cc. of a moderately 

 rich sample of tow-netting. The experiments were made during the 

 spring and summer months, and the general course of events has been 

 the same, with a certain amount of difference in detail according to the 

 nature of the plankton present at the time. 



During the first two days the water often became cloudy, owing to 

 the rapid multiplication of small fiagellate infusoria, though this was 

 not always the case. Plankton Copepods and other animals gradually 

 died off, though some survived for as long as a week or ten days. The 

 plankton diatoms, on the other hand, generally multiplied rapidly 

 during the early days of the experiments, the first to become abundant 

 in the body of the water being usually Shdetonema costatvm, which at 

 the end of a week might be so thick, that a number of chains could be 

 seen in every drop of water examined with the microscope. Along 

 v^rith the /'^keletonema were found other plankton diatoms, such as 

 Lauderia horealis, Chaetoceras (two or three species), BidduljjlLia mobi- 

 licnsis, Diti/liiim B right wellii, and in nearly every case Thalassiosira 

 ■dccipiens. These latter diatoms were present in moderate numbers 

 only, when the Skcldoncma was at its height ; but as the Slxlctoiiema 

 •died down they increased in quantity. At the same time Nitzsclda 

 closterium commenced to appear, both amongst the precipitate on the 

 bottom of the jar, and in the general body of the water. Small green 

 llagellates often began to get numerous also at this stage. The true 

 p)lankton diatoms were at their height about a fortnight after the 

 -experiments were started. At this time a great many diatoms of all 



