The shelly tubes of these animals are built in the form of serpents, or twisted funnels, of a milk-white colour. Although so extremely hard, these tubes are formed solely by an exudation from the body of the animal--a simple marine worm. Unlike its erratic friend, the earth-worm, the Serpula is sedentary in its habits, and at no time does it ever leave its dwelling. The delicate, but brilliant feathery plume--the only portion of the animal ever visible--constitutes the principal mechanism by means of which the Serpula constructs its calcareous tube. A most wonderful instance of how mighty are the works which these insignificant creatures form when congregated together in vast numbers, and how useful such labours may sometimes be to mankind, is narrated by Dr. Darwin in his 'Voyage of the Beagle.' Being delayed by adverse winds, this gentleman made a stay at Pernambuco, a large city on the coast of Brazil, and the most curious object that he saw there was the reef that formed the harbour. 'I doubt,' to use his own words, 'whether in the whole world any other natural structure has so artificial an appearance. It runs for a length of several miles in an absolutely straight line, and parallel to, and not far distant from the shore. It varies in width from thirty to sixty yards, and its surface is level and smooth; it is composed of obscurely stratified hard sandstone. At high water the waves break over it; at low water its summit is left dry, and it might then be mistaken for a breakwater erected by Cyclopean workmen. On this coast the currents of the sea tend to throw up in front of the land long spits and bars of loose sand, and on one of these the town of Pernambuco stands. In former times a long spit of this nature seems to have become consolidated by the percolation of calcareous matter, and afterwards to have been gradually upheaved, the outer and loose parts during the process having been worn away by the action of the sea, and the solid nucleus left as we now see it. Although night and day the waves of the open Atlantic, turbid with sediment, are driven against the steep outside edges of this wall of stone, yet the oldest pilots know of no tradition of any change in its appearance. This durability is by far the most curious fact in its history; _it is due to a tough layer, a few inches thick, of calcareous matter, wholly formed by the successive growth and death of the small shells of Serpulæ, together with some few Barnacles_, &c. These insignificant organic beings, especially the Serpulæ, have done good service to the people of Pernambuco, for without their protective aid the bar of sandstone would inevitably have been long ago worn away, and without the bar there would have been no harbour,'