The facts recorded in the following note were ascertained in the course of my observations made upon a marine rock-boring polychæte worm found at Tenby in the spring of the present year. This worm, a species of Potamilla, is living in limestone rock, in which another species of the same genus, Potamilla reniformis , is also burrowing. It differs from the latter in various particulars, amongst which may be noted the absence of eyes on the branchial filaments, the colour of the blood (which is red instead of being sometimes green), the form of the setæ, and the character of the external tube, which is largely covered, especially at the tip, with minute pieces of shell attached edgewise, imparting to it a white, rugged appearance, somewhat similar to that of the tube of Owenia . The worm is sometimes as much as 3½ inches long, and is seldom extracted entire from the rock, fragments only, of varying length, usually being obtained. It occurred to me that this material might be utilised for the study of the regeneration of the lost parts, and my experiments in that direction succeeded beyond my expectations. Not only did the fragments renew these parts (both anterior and posterior) but they demonstrated the existence of a power to economise labour in this respect, by changing the arrangements of certain of the old parts , so as to complete the model of the original animal.