Still more remarkable is the case of the Double-eyed Fish (_Anableps_). In this fish there is an intromittent organ in the shape of a tube which is formed by a continuation of the urinogenital ducts down the front of the anal fin. In the hinder half of this organ a bend is made either to the right or left. Out of seventeen males, this bend was to the right in eleven, to the left in six. Further, there is a small fleshy tubercle at the side of the anal fin-ray, at the middle of its length. When this prominence is on the left side, the organ bends to the right; when it is on the right, the bend is to the left. In the females the genital opening is covered by a special scale, which is free on one side, left or right, and not on the other. Thus copulation is possible only from the side, and a left-sided male can only conjugate with a right-sided female, and vice versa. Here is one of the most extraordinary cases of specialized secondary sexual characters known. How do the sexes distinguish their complemental mates? It is important that they should, for unions are otherwise impossible. In the Dragonet (_Callionymus lyra_) the male differs conspicuously from the female in being much the larger—an exception to the rule—and in having the fin-rays enormously elongated. Further he wears a conspicuously resplendent livery, but this is strictly a “nuptial” livery, the colours waning as soon as the period of sexual activity is past. That these colours play the same part as with the birds is clear from the observations of the late Saville Kent. “The male,” he says, “resplendent in his bridal livery, swims leisurely round the female, who is reclining quietly on the sand, his opercula distended, his glittering dorsal fins erect and his every effort being concentrated upon the endeavour to attract the attention of his mate.... The female, at first indifferent, becomes at length evidently dazzled by his resplendent attire and the persistency of his wooing. She rises to meet him, the pair so—far as is practicable with fishes—rush into each other’s arms, and with their ventral areas closely applied, ascend perpendicularly towards the surface of the water.” In the course of this ascent the ova and sperms are shed, and fertilization takes place.