The Sea-Mouse, one of the largest and commonest of our marine worms, is of a flattened and somewhat oval form, pointed at each end, its general colour being pale brown. The clothing of silky hairs, however, with which it is invested, is so splendid, glittering in iridescent colours like the plumage of a humming-bird, that Lamark has appropriately named it _Halithea_, or Sea-Goddess—Linnæus having previously given it the name of _Aphrodite_, the Marine Venus. When, indeed, it receives the rays of light, and reflects them from the depths of the sea, rich with prismatic hues, the effect has been compared in splendour to that of the peacock’s tail when outspread in the sunshine. When in the Aquarium it crawls restlessly to and fro, as though anxious to exhibit its splendours in every possible point of view; the metallic tinges, changing with every position, being most magnificent by candle-light, when red and orange hues predominate, while by day-light pearly greens and blues are most frequent. The bodies of many of the _Euricidæ_ and _Nereidæ_ exhibit changing colours of similar character, though less splendid; but many of them possess an opal-like tenderness and delicacy almost equally attractive. The little Sea-Slug, _Ægines punctiluceus_, is a brilliant little creature, well worthy the trouble of being permanently established in our Aquaria. Its general colour is pale reddish-brown; but, with the aid of a moderate lens, it exhibits a number of small black tubercles, in the centre of each of which is a speck of resplendent blue or green, forming a succession of gem-like ornaments that have been compared, by enthusiastic naturalists, to sapphires and emeralds; but, without exaggeration, the jewelry of this little creature may be said to surpass that of the Diamond Beetle, notwithstanding his superior reputation. A few _Chitons_—a sort of Sea Wood-Louse—would do well in a tank; and a specimen of _Gastrophæna modolina_ is said to have thriven well during many months. [Illustration] ------------------------------------------------------------------------ CHAPTER VIII. THE FISH AND CRUSTACEANS OF THE AQUARIUM.