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 : — Off the Colne . 1026-87 Off Clacton and Walton . . 102673 Near the Cork Light . 1026-37 Off Southwold .... • 1025-53 Off Lowestoft ..... . 1024-57 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 87
