A HISTORY OF SUFFOLK Essex and Suffolk it also seems to show why some are good for the culture of oysters and others unfit, and why they sometimes turn green and in other estuaries lose greenness acquired elsewhere. Marine and Freshwater Plankton When carrying out researches in the Thames in 1882 in connexion with the main drainage of London on behalf of the Board of Works it occurred to me that it was desirable to ascertain the number per gallon of water of such small animals as Cyclops, since their excrements were often so abundant in the mud as to indicate that they might play an important part in destroying the sewage material. Subsequently from the early part of May to the end of September I systematically and regu- larly carried out similar inquiries in the various estuaries and rivers of Suffolk, Essex, and Kent, and in the more open seawater off the coast. At that time little or no attention had been paid to this subject, which is now looked upon as very important because such small animals form so large a part of the food of larger species. By the method adopted I obtained in a small bulk of water all the living animals too of an inch or more in diameter in a known volume of water, collected near the surface, near the bottom, and half way between. I found it most convenient to give the number per 10 gallons in order to avoid fractions when dealing with mean results, though as a general rule only 2J gallons were collected. No attempt was made to distinguish the species, which would have made the study much more difficult and com- plicated, but the different groups of animals could easily be recognized by their manner of swimming. The numbers thus found varied enor- mously from 10 gallons, in fact from only two or three up to 10,000 or more. One object which I had in view was to ascertain the influence of the varying amount of salts on the kind and number of the small free- swimming animals in passing down the rivers from where the water was fresh till we come to the sea water itself. This is a question of paramount importance in studying estuaries, which in the case of Suffolk have so great an extension. At the same time the results depend not merely on the amount of salts, but also on associated conditions of different kinds. In order therefore to properly elucidate the facts I determined the excess in the weight of the volume of 1,000 grains of rain water.' The rivers and estuaries I studied in 1884 were: in Suffolk the Aide, Butley, Orwell, and Stour ; in Essex the Colne, Blackwater, Crouche, and Roche; and in Kent the Medway. Since 1884 I have much extended these observations. ' The late Dr. H. C. Sorby evidently intended to give tables showing the distribution of animals similar to those printed in F.C.H. Essex, i, 74, but no details on this point have been found among the MS. notes relating to his article [Editor V.C.H.]. R8
