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 



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