The washing seems to remove excreta of the muscle's own production, and the period of repose removes them perhaps by diffusion, perhaps by breaking them down into innocuous material.
The water which bears the oxygen for respiration and the minute organisms upon which the Brachiopod feeds is swept into the mantle cavity by the action of the cilia which cover the arms, and the eggs and excreta pass out into the same cavity.
The excreta of urea alone thus afford to the soil enormous stores of nitrogen combined in a form which can be rendered available by bacteria, and there are in addition the supplies brought down in rain from the atmosphere, and those due to other living debris.
Peruvian guano is obtained from the excreta of South American sea-birds, and fish guano from the waste of fish.
Now dead animal substance and the excreta of animals decompose in the long run into carbonic acid, water and mineral salts, and so there is a continual destruction of animal substance both on the land and in the sea.