202 THE ZOOLOGIST. 
Oystercatchers spend the time of high-water resting near the 
high-water mark. They fly over to the scalps exposed by the 
ebb, and occupy themselves with the uncovered shell-fish ; as the 
scalps become dry the birds turn their attention to the edges of 
the banks, the adjacent sand or mud, and the pools in quest of 
hidden Mussels. The period of slack water is devoted to repose, 
or is spent in other ways, and during the rise and flow of the tide 
over the scalps the Oystercatchers renew the search, until they 
are carried literally off their feet by the flood, when they betake 
themselves once more to the high-water mark. There is reason 
to believe that they are able to search effectively in the dark, and 
they are certainly active on moonlit nights. 
The preceding paragraph shows that the Mussels must be 
surrounded by a certain amount of moisture if they are to meet 
the requirements of the Oystercatchers. With one exception, 
to which reference will be made, the Mussels which lie on the 
surface of the scalps are left alone when once they have become 
dry, and attention is concentrated on those which are covered 
by seaweed or by water, and on those which are buried in sand 
or mud. 
The attitude of rest is one in which the valves of the Mussel 
are separated slightly along the free border, due to the tension of 
the elastic ligament; the tight closure of the valves is caused by 
the contraction of the adductor muscles, and its maintenance 
implies continuous exertion. The attitude of rest is possible 
only when the shells are under water or in moist situations ; 
otherwise the delicate internal structures would shrivel. As soon 
as the shells become dry the Mussels must close their valves. 
Hence it is found that in the one case the shells are gaping 
slightly, in the other they are tightly closed. The former are 
liable to destruction by the Oystercatchers; the latter, with one 
exception, as far as can be discovered, are invulnerable. It is 
essential that the moisture should be saline. Heavy rainstorms 
interfere with the search of the Oystercatchers by flooding the 
scalps with fresh water, which has the same effect as the drying 
of the shells. 
Inspection of any scalps on which Oystercatchers have been 
feeding shows a litter of emptied shells. Some still lie in posi- 
tion on the scalps, others remain at the bottom of conical 
