MARINE ZOOLOGY excess of density is about 23 or 24. This fact is of much interest in connection with the culture of oysters, which is such an impor- tant industry in Essex, since it is in such localities that they are laid down to fatten ; and although they probably do not feed on some of the animals included in the table, it seems almost certain that there is a corresponding excess of the more minute animals and plants on which they do feed. The relatively great number of larval Balani and Ascidians is also to be noticed, and agrees with the fact that the bottoms of vessels become so much covered with them when lying long in such water. It will also be seen that there is a well marked secondary maximum where the excess of density is about I2|, due almost entirely to the great development of Copepoda. Table II. shows the distribution of some of the minute animals from the top to the bottom, and also according to the state of the tide. Table III. shows what a remarkable increase occurs in the number of free-swimming animals as we proceed from clear running rivers to more stagnant water, in which they have time to multiply. This fact was confirmed in a striking manner by my observations in the rivers and broads of Norfolk. As will be seen, the mean number in the sheltered estuaries and the more open water just outside them is comparable to that in the stagnant freshwater ponds, probably because it also is not so changed, by the introduction of plankton-free water, as to prevent the accumulation of the animals. THAMES In August and September of 1882 I lived at anchor for about six weeks at Greenhithe and Erith, carrying out researches for the Board of Works in connection with the main drainage of London. For this purpose it seemed important to study the Entomostraca and the free- swimming organisms, since the combination of various facts indicated that minute animals and plants played a very, important part in removing sewage from the river. Though the Thames in the district studied cannot be looked upon as sea water, yet it forms such a long stretch of coast in Essex that it seems desirable to describe some of the observed facts, it being of course understood that they relate only to a particular season of the year. The extraordinary strength of the tidal currents, and the great amount of mud held in suspension, make this river very unlike any other in Essex. Table IV. shows the mean number of Entomostraca and larval shrimps in 10 gallons of water, combining half flood with high water and half ebb with low water, and shows well the variation in the num- bers of these animals which are more especially characteristic of fresh water and salt water, the extreme variations being at Erith combined half ebb and low, and at Greenhithe combined half flood and high. One effect of this great and constant change in the character of the water 75
