Sea water from the Plymouth district and sea water from the Firth of Clyde each produced, from the same batches of fertilized eggs of Echinus esculentus, a different morphological type of larva. Each type was clearly recognizable irrespective of the copper concentration up to a level higher than that ever likely to be found naturally in the localities from which sea waters contrasted in earlier experiments had been obtained. Suggestions by continental workers that these earlier results may have been due to varying copper concentration are thus shown to be inapplicable.