The animal is a voluntary life prisoner, for the aperture has the least dimension of the tunnel. The genus is known to be self luminous--a decided advantage in so dark and narrow an habitation. It seems to me to be worthy of special note that an animal enclosed by Nature in tightly fitting valves should also be endowed with the power of mixing plaster or secreting the enamel with which its tunnel is lined and of depositing it with like regularity and, smoothness to that exhibited in its more personal covering which grows with its growth. The mollusc in its burrow in the depths of a block of coral, white as marble, with its own light and its self-constructed independent wall, appeals to my mind as evidence of the care of Nature for the preservation of types, while from such retiring yet virile creatures man learns earth-shifting lessons. A quotation from Lyell's "Principles of Geology" says that the perforations of Lithophagi in limestone cliffs and in the three upright columns of the Temple of Jupiter Serapis at Puzzuoli afford conclusive evidence of changes in the level of sea-coasts in modern times--the borings of the mollusc prove that the pillars of the temple must have been depressed to a corresponding depth in the sea, and to have been raised up again without losing their perpendicularity. The date-mussels play an important part in the conversion of sea-contained minerals into dry land. Massive blocks of lime secreted by coral polyps being weakened by the tunnels of the mussels are the more easily broken by wave force; and being reduced finally to mud, the lime, in association with sand and other constituents, forms solid rock. A feature of another of the coral rock disintegrating agents is its extreme weakness. It is a rotund mollusc with frail white valves, closely fitting the cavity in which it lives. As it cannot revolve, the excavation of the cavity is, possibly, effected by persistent but necessarily extremely slight "play" of the valves; but the animal appears to be quite content in its cramped cell with a tiny circular aperture (generally so obscured as to be invisible), through which it accepts the doles of the teeming, incessant sea. CHAPTER XV BARRIER REEF CRABS "Reasoning, oft admire How Nature, wise and frugal, could commit Such dispositions with superfluous hand." MILTON.