Well, the reason is a very odd one. It is setting a trap for fishes! For fishes, as perhaps you know, are very inquisitive creatures. They always want to know all about everything, and whenever they see a hole they think that they must find out what is inside it. So when a little fish comes swimming past a pinna, and catches sight of its gaping shells, it is almost sure to venture in between them. Then the shells close tightly, and it finds itself in a prison from which there is no escape; and very soon it is killed and devoured.

In colour, the shells of the pinna are very pale brown, and a number of ridges run down it from the smaller end to the larger. When the animal is full-grown it is sometimes not at all easy to see its shells, for they are covered almost all over with barnacles and the tubes of sea-worms.