covered with bristles, thus greatly differing in appearance from most worms. The larger bristles, which are of a dark purplish-black colour, have a bronze or golden metallic lustre. Various other annelids exhibit brilliant rainbow colours; for example, _Nereis_, the Rainbow Worm, also found on English shores; but without the underlying black opaque pigment, the reflections from the surface fall short of absolutely metallic brightness. On land, we see among the insects innumerable forms which present a metallic lustre, the beetles being the most notable in this respect. To return to the vertebrates, from which we started, everybody must have noticed that the fur of a clean well-kept black cat, when lit up by the bright sunlight in which the animal loves to bask, shows little rainbow reflections of red and green. These are due to the presence of little grooves and irregularities on the surface of the hairs, which play the same part in breaking up the light which they reflect, as do the sharp angles of iridescent feathers. Like the iridescence of the Rainbow Worm, they fall short of absolutely metallic brightness; the fault in this case being due not to the nature of the underlying stratum, so much as to the incomplete development of the light-reflecting grooves. Yet this instance serves to show the part taken by the dark pigment; for while the play of colours is perfectly obvious in the fur of a black cat, it is almost impossible to distinguish it in the case of cats with fur of lighter shades. The Mammalia, or animals that suckle their young and produce them by birth, were formerly considered to be sharply defined from animals that lay eggs, such as the birds and reptiles. But in 1884 Mr. Caldwell confirmed the statement which had been made previously, yet hardly credited by the scientific world, to the effect that the lowest form of mammals lays eggs. This, the Duck-Mole or _Ornithorhyncus anatinus_ (Bird-billed animal much like a goose), is a native of Australia and Tasmania. It lives on the banks of rivers, and burrows in the bank. It has webbed feet, and therefore sometimes receives the name of Platypus (flat-foot). It lays eggs two at a time, in its burrow; and these eggs, like those of other egg-laying vertebrates, have a yolk.