A HISTORY OF KENT 



the inner into the outer harbour. In 1882 the surface was covered 

 with them, and among the most striking were various species or 

 varieties of Botryllus. These were not identified at the time, and the 

 few specimens preserved in the Public Museum at Sheffield could not now 

 be identified in a satisfactory manner, but are probably Botryllus schlosseri 

 and polycyclus. Botrylloides rubrum (M. Edw.) or a closely allied species, 

 and very fine specimens of Polyclinum auranthim also occurred. Of 

 simple ascidians Ascidiella aspersa (O. F. Miiller), and I think a few in- 

 dividuals of yf.T;/r§-/>;ra (O.F. M.) were found, and Cionaintestinalis (Linn), 

 of larger size than any seen in Essex or Suffolk, those 6 inches long 

 being common. 



Straits of Dover. 



On one occasion I dredged between Dover and Boulogne, and 

 found many fine specimens of Alcyoiuum digitatwn (Linn.) and many 

 sponges, chiefly if not entirely, Halichondria panicea (Pallas). 



The Thames at Greenhithe and Erith. 



In 1882 and 1883 I spent a number of weeks on my yacht off 

 Greenhithe and Erith, studying the water of the Thames on behalf of 

 the Board of Works. The greater part of my observations have only 

 an indirect connexion with marine biology, and I describe only a 

 few of my results. Much depends on the very strong tide, which 

 causes a great difference in the character of the water at different times 

 of day, which alternates between fresh and very salt. Few or no 

 sedentary animals are able to live under such conditions. Besides 

 this in some places the material deposited at the bottom at the period 

 of the neap tides is carried away by the much stronger current of the 

 spring tides. The living animals swimming in the water, of which 

 I determined the number per gallon, were those which move up and 

 down in the water, and at high tide many small shrimps were met with, 

 whereas the low water is mainly characterized by the presence of such 

 forms as Cyclops and Daphne. Gammarus occurs in about equal numbers 

 in high and low water. As bearing on the removal of sewage from the 

 water by Copepoda I may say that I found they lived about six times as 

 long when small quantities of human excrement were added to the water 

 as when none was added. Those living in the low water soon died in 

 the high water, and one part of sea water added to two or three of fresh 

 soon proved fatal, which explains what I observed where semi-marine 

 conditions occur along the northern shore of Kent. 



The Medway. 



In passing down the Medway from AUington to Sheerness, the 

 conditions are much modified by the great extent to which the tide 

 runs out in the upper part; and the amount of salt in the water where 

 a well-marked maximum in the number of Copepoda occurs appears not 

 to be the same as in the estuaries in Essex and Suffolk. 



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