MARINE ZOOLOGY 



exposed, and the sand and shingle are little suited for collecting marine 

 animals as they are so apt to be washed away or covered up. I know 

 little of the coast except near Dunwich and Southwold, but both there 

 and at Lowestoft the shore looked so unpromising that I never attempted 

 to dredge, which, moreover, would have been very inconvenient when 

 living on a yacht since there is no available harbour except Lowestoft. 



Density of the Water 



The amount of various soluble salts as measured by the density of 

 the water has often such a preponderating influence on the local distri- 

 bution of many marine animals that it is important to consider it specially. 

 It even seems to have a decided effect when the difference is com- 

 paratively small. My observations have extended over a considerable 

 area and for a number of years, but have all been made in the summer 

 months from May to September inclusive, just at the time when the 

 distribution of the animals was studied. In the cold part of the year 

 nearly everything would be different. 



First with regard to the more open water along the coast. My 

 observations north-west and south-east of Harwich have been sufficiently 

 numerous, but north of Orford Ness have been confined to only one 

 season, viz. 1886. There is a very decided difference in the means of 

 different years and therefore I compare the densities for that year alone. 

 I then found that the density gradually decreased from the Wallet off 

 the Colne to the North Sea off Lowestoft as follows : — 



The explanation of this probably is that at the south-west end there is so 

 much more shallow water exposed to evaporation, while at the north- 

 east end the great bulk is of the deeper water of the North Sea. It is, 

 however, very desirable that this conclusion should be confirmed by 

 observations in other years. Judging from what I have learned in the 

 estuaries, such a difference is quite sufficient to materially influence the 

 local distribution of some marine animals. 



In comparing the various estuaries one of the most important 

 points to consider is the difference between the mean density of the high 

 and low water in different parts. This alone is sometimes quite sufficient 

 to explain why the animals differ. This difference in density at the 

 lower parts of an estuary depends on a considerable number of circum- 

 stances, and may be looked upon as a very good index of the general 

 character of the species found there. To enter into detail would be out 

 of place here, but I may say that the extent of this difference seems to 

 go far to explain the local distribution of many animals as shown in 

 my lists, and on comparing the different estuaries along the coast of 



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