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 MARINE ZOOLOGY Crustacea or the Mollusca. Those of Essex are from my own list, including those collected by Mr. Hope and Mr. Unthank : In Essex In both Essex and Norfolk In Norfolk Hydrozoa 17 1-7 2Q Actinia I 2 I i 2 Medusae 6 2 2 Echinoderms 7 7 / 2 I 2 Chztopoda 28 21 Polyzoa 22 c 14 Simple Ascidians .... 9 97 2 39 5 86 It will thus be seen that there is a remarkable difference, and that whilst fifty-eight species found in Essex are not found in Norfolk, and forty-seven in Norfolk not in Essex, there are only thirty-nine common to both. These results are no doubt in part due to imperfect collecting, but yet, I think, still more to the materially different characters of the coasts. MARINE AND FRESHWATER PLANKTON During the years 1882 to 1889, from the early part of May to the middle of September, I carried out continuous observations of the num- ber of small free-swimming animals per gallon of water. The method adopted was to collect the water near the surface, half way down and near the bottom, in a bottle sunk empty, and filled by pulling out the cork. Usually 2 gallons were collected, and passed through a fine sieve, not allowing any animal larger than ^ of an inch in diameter to pass. The material so separated was then washed out by pouring a few ounces of sea water on the back of the sieve, care being taken that none of the animals should be killed by drying. I had a glass trough 3 inches deep, i inch broad, and | inch from front to back, into which the water containing the animals was poured, and the number of the various kinds counted, the process being repeated until the whole had been examined. The different sorts could in general be easily dis- tinguished by their different form or manner of swimming. It would be out of place to go into detail, but it seems desirable to give some of the general results which, though never printed, were repeatedly described by me in public lectures at a time when this subject had attracted little or no attention. In the following tables I give the number of the animals per 10 gallons of water, but omit those which on an average do not amount to one in that quantity. These were determined from my first three years' observations, and might be somewhat modified by the later, which have not yet been adequately discussed. As might be expected larval forms play a very important part. 73 10