An Ancestral Habit in a Sea-Urchin

Abstract

THE heart-urchin Echinocardium cordatum is one of the most abundant species in the littoral fauna of the British Isles. It is found everywhere where smooth sandy beaches occur and it is distributed all over the bottom of the North Sea to the Danish coast. Its normal mode of life is to excavate a burrow for itself situated about 6 inches below the surface of the sand: this burrow is connected by a vertical shaft with the surface. The roof of the burrow is supported by a cockscomb-like crest of curved spines and the surface of the urchin is quite unpolluted by the sand which forms the wall of the burrow. Through the vertical shaft the urchin protrudes the long tube-feet which belong to the anterior ambulacrum: the discs terminating these tube-feet are fringed with fingers so as to resemble small sea-anemones and with these the urchin sweeps up small animals lying on the bottom. So effective are they, that where Echinocardium abounds no mussels can exist: for as soon as the young mussels metamorphose from free-swimming larvae, they are seized by the tube-feet of the buried Echinocardium and conveyed to its mouth.