Although students of conchology are familiar with several species of LIMA, I am eager to include it in these haphazard references, because my first acquaintance with a living specimen afforded yet another experience of the versatility of the designs of Nature. It is truly one of the "strange fellows" which Nature in her time has framed. Living obscurely in cavities, under stones, inoffensive and humble, the Lima enjoys the distinction of being, the permanent exemplification of the misfit, its body being several sizes too large as well as too robust for its fragile, shelly covering. The valves are obtusely oblong, while the animal is almost a flattened oval, the mantle being fringed with numerous bright pink tentacles, almost electrical in their sensitiveness.
Though anything but rotund, so full in habit (comparatively speaking) is the body of the lima that the valves cannot compress it. Except at the hinges they are for ever divorced, an unfair proportion of the bulging body being exposed naked to the inclemency and hostility of the world. "All too full in the bud" for those frail unpuritanical stays, the animal seems to be at a palpable disadvantage in the battle of life, yet the lima is equipped with special apparatus for the maintenance of its right to live. By the expansion and partial closing of the valves it swims or is propelled with a curiously energetic, fussy, mechanical action, while the ever-active pink rays—a living, nimbus—beat rhythmically, imperiously waving intruders off the track.
The appearance and activities of the creature are such as to establish the delusion that it is not altogether amicable in its attitude towards even such a bumptious and authoritative product of Nature as man. Its agitated demonstrations—whatever their vital purpose may be—to the superficial observer are danger signals, a means of self-preservation, as a substitute for the hard calcareous armour bestowed upon other molluscs. The fussy red rays may impose upon enemies a sense of discretion which constrains them to avoid the lima, which, though hostile in appearance, is one of the mildest of creatures. The tentacles, too, have a certain sort of independence, for they occasionally separate themselves from the animal upon the touch of man, adhering to the fingers, while maintaining harmonic action, just as the tip of a lizard's tail wriggles and squirms after severance.
Most of the blocks of submerged, denuded coral are the homes of certain species of burrowing molluscs, the most notable of which are the "date mussels" (LITHOPHAGA). The adult of that designated L. TERES is over two inches long and half an inch in diameter; glossy black, with the surface delicately sculptured in wavy lines; the interior nacreous, with a bluish tinge. This excavates a perfectly cylindrical tunnel, upon the sides of which are exposed the stellar structure of the coral. A closely related species (STRAMINEA), slightly longer, and generally of smooth exterior, partially coated with plaster, muddy grey in colour, adds to the comfort and security of existence by lining its tunnel with a smooth material, a distinction which cannot fail to impress the observer. In each case the mollusc is a loose fit in its burrow, having ample room for rotation, but the aperture of the latter is what is known as a cassinian oval, and generally projects slightly above the surface of the coral.
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."