Sometimes they are thick and hard, so as to refuse the passage of light. Sometimes they are so thin as to be translucent. Sometimes they are exquisitely pearly or iridescent. The latter effect, seen in the mother-of-pearl lining of oyster shells, is due to enormous numbers of most fine and delicate lines, close together, which reflect and break up the rays of sunlight, much after the manner of a prism. Perhaps in the whole Mollusc Family none of the members are so interesting to man as the oyster--partly as a much-relished food, partly as the manufacturer of pearls. Among gems worn by women few are fairer, none more emblematic of purity and grace, than these. Yet, so far as relates to any intention on the part of the oyster, any idea of forming that which is lovely and valuable, no thanks whatever are due to the producer. An oyster makes pearls simply for its own convenience. As in the case of other bivalves, if any foreign substance happens to get inside the shell, and cannot be pushed out, it is straightway covered over, and thereby rendered harmless. Sometimes the intruder is a grain of sand, sometimes a parasite. Oftener yet, it is an oyster-egg which has proved a failure. In any event, the oyster promptly clothes it in that beautiful material--pearl--which it secretes by nature, and because it cannot do otherwise. We know well enough the ordinary size of “edible” oysters. The pearl-forming relatives are often much larger, having shells from eight to twelve inches in diameter. One does not commonly associate the idea of Mind with a Limpet. Yet it appears that even limpets are not without the organ of locality. Human beings, brilliant in other respects, are sometimes woefully deficient in this quality. But a limpet knows what he is about as to the geography of his own locality. He chooses his resting-place with care, much preferring a smooth rock-surface to one that is broken or dented. Then he clings fast, and the shell in its growth is studiously adjusted to the outlines of that selected surface. When the limpet “excursionises” in search of sea-weeds, vegetables being a needful part of his diet, it is remarkable that he returns to the same spot. Evidently he prefers his own home to new quarters. These lower Molluscs have to be passed over quickly. A whole volume might be filled in merely indicating the different species and varieties of them. At the top of the Mollusc ladder, cousin to limpets, whelks, oysters, clams, and shell-fish innumerable, but in status far above them all, we find the powerful Cuttlefish. A “fish” in popular language; yet no fish truly, but a very highly developed Mollusc. “Cephalopod” or “Head-footed” is the distinguishing name for this sub-class of Molluscs. Very unpleasant creatures they are to tackle--more particularly those described by the familiar titles of “Octopus,” “Squid,” and “Devil-fish.” More particularly too when an individual of large dimensions has to be reckoned with.