Look, again, at those sea-slugs. One, some three inches long, of a bright lemon-yellow, clouded with purple; another of a dingy gray (Doris tuberculata and bilineata); another exquisite little creature of a pearly French white, furred all over the back with what seem arms, but are really gills, of ringed white and gray and black (Eolis papilosa). Put that yellow one into water, and from his head, above the eyes, arise two serrated horns, while from the after part of his back springs a circular Prince-of-Wales’s-feather of gills--they are exactly like those which we saw just now in the white Cucumaria. Yes; here is another instance of the same custom of repetition. The Cucumaria is a low radiate animal--the sea-slug a far higher mollusk; and every organ within him is formed on a different type; as, indeed, are those seemingly identical gills, if you come to examine them under the microscope, having to oxygenate fluids of a very different and complicated kind; and, moreover, the Cucumaria’s gills were put round his mouth, the Doris’s feathers round the other extremity; that gray Eolis’s again, are simple clubs, scattered over his whole back, and in each of his nudibranch congeners these same gills take some new and fantastic form; in Melibæa those clubs are covered with warts; in Scyllæa, with tufted bouquets; in the beautiful Antiopa they are transparent bags; and in many other English species they take every conceivable form of leaf, tree, flower, and branch, bedecked with every color of the rainbow, as you may see them depicted in Messrs. Alder and Hancock’s unrivaled _Monograph on the Nudibranch Mollusca_.