The paradox exists, not as a whim or grimace on the part of Nature but for a definite and vital end. In default the animal would be unable to obey the first law of Nature--self preservation--for it is soft-bodied and its dwelling has the serious defect of being open at both ends. In such plight lacking special organs it would be at cruel advantage in the struggle for existence. The posterior segment of the body is therefore developed into an operculum-like organ, smooth and of horny texture, which closes the narrow end of the tube. The other extremity is more elaborately guarded, the anterior segment being fringed with a frontal membrane, while the second segment forms a disc, the minute mouth orifice with the true tentacles and gills being debased to the third segment. Confronted by danger, the animal closes its front door by retracting until the disc presses immovably against the circumference of the tube, the retraction being so sudden that a frail spurt betrays the whereabouts of an otherwise secret dwelling-place. In the centre of the disc is the first segment, from which the frontal fringe is extended in the form of an array of keen bristles as a defensive weapon. With the lid at one end and the armed disc at the other the animal enjoys security and comfort, and when unsuspicious the "shoulders" protrude, the head meekly following. The tentacles are serrate and glitter like tinsel, possibly for the fascination of the minute forms of life which the tube-dweller consumes. To enable it to retract and emerge quickly the animal is provided with a series of tufts of bristles on the back and on the ventral surface of the body with a row of toothed "pads," which fulfil the dual office of grapplers and feet. With what skill and patience does this pectinarian construct its ornate habitation! How artfully does it pick and choose among the tiny shells and grit! With what rare discretion rejects the unfit, and with what satisfaction retains a neat and dainty item of building material! How deftly does it arrange its courses and bonds, cementing each fragment in its place until a perfect cylinder, proportionate in dimensions, uniformly expanding in circumference, smooth within, rugged without, scientifically correct in design, is the result! How apt, too, the frictionless lining! And all this laborious neatness and precision absorbed in the construction of a tenement which has no time! Does the inmate possess any sense of duration? Addison (quoting a French authority) says that it is possible some creatures may think half an hour as long as we do a thousand years! The magnificent mind of the modern biologist regards a million of years as a mere fag-end of time. The industrious worm which has built so choice a home may have enjoyed the sense of comfort and security for a period representing an honourable age, while, according to the standards of man, the home was not worth the building for so short a tenancy.