The Lamellibranch, or bivalve Mollusca, _e.g._, Oyster, Mussel, and Cockle, afford valuable evidence as to excrescences and extravagances of growth which appeal to our eyes as ornamental, and therefore likely to be due to the influence of sexual selection. And this because such ornamentation is a very conspicuous feature among these animals. Yet, save in a few cases, locomotion is impossible, and sight is wanting. Light-distinguishing organs, and therefore eyes, are possessed by some, but in no case probably are they strong enough to appreciate form. Even if they did, such revelations of beauty would play no part in mate selection from among the most ornamental; for these creatures are commonly fixed throughout life in one position, often, indeed, buried in mud or sand. Some move laboriously: a few, like the Cockles and Pectens, swim by rapidly opening and closing the shell. The Pectens are brilliantly coloured, not only as regards the shell, which is also beautifully sculptured, but the foot also is of a vivid scarlet, and the Pecten have numerous minute eyes. But the Cockles and Mussels possess like attributes as to colour and sculpture, yet they are blind. More to the point is the fact that these animals do not mate after the fashion of higher animals, but the males, where the sexes are distinct, discharge immense quantities of spermatozoa into the water, and these find their way to the ova of the female through the action of the inhalent currents set up by the animal for the purpose of drawing in fresh supplies of water containing food and oxygen. There are no “secondary sexual characters,” that is to say, that even where the sexes are separate, and many, like the Oysters, are hermaphrodite, they are externally indistinguishable. Nevertheless, many, as has been already remarked, have shells of great beauty. As, for example, the giant Tridacna and the strangely spinous valves of the “Thorny Oysters” (_Spondylidæ_).