It is well known that the ink of the Chinese, which we term Indian ink, is prepared from the Cuttle, and the Cuttles of the Mediterranean Sea furnished the principal black inks and dyes of the Greeks and Romans. It is a kind of _Sepia_, in fact, that still furnishes the rich brown colour which bears the name of the animal from which it is derived. The common _Sepia vulgaris_ might form an appropriate specimen for a marine Aquarium, many of its habits being singular; and its power of enveloping itself in a cloud of its own rich dye may often be observed when it is irritated by the presence of a real or fancied danger. It has the faculty of propelling itself hither or thither by the emission of a jet of water, as described in speaking of the locomotive power of other Molluscs, with the additional faculty of guiding its motions by the rapid movement of two fin-like paddles, which, when in agitation, produce an effect not unlike the fluttering action of the wings of a moth. This little _Cephalopod_ has large projecting eyes, and a group of arms that hang listlessly down when the fins are in motion. It changes colour fitfully and beautifully, exhibiting in the course of such changes pretty metallic spots and rings, which appear and disappear, now like gold, now like silver, as seen through a semi-opaque substance. The whole creature is at one moment of a dusky gray tone, but fitfully changes to white or deep brown when alarmed. These little creatures are exceedingly voracious, and when one was observed by Mr. Gosse to seize another of its own species, the victim shot out its defence of dark black fluid. Some of the _Trochus_ tribe of shells look pretty in an Aquarium, but at present their treatment is so imperfectly understood, that they do not seem to do well. The specimens can, however, be renewed as required. _Trochus ziziphinus_, the pearly _Trochus_, the animal of which is of a rich orange colour striped with black, moves freely about, and forms a very attractive object. The animal of _T. granulatus_ is larger and handsomer, but shy, and displays little activity in confinement. The small _Trochus_, _T. cinerarius_, if placed in an Aquarium, may be observed rasping down the minute Confervæ that grow on the inside of the glass; and the curious method of the operation, and the singular instruments with which it is performed, may be observed by the aid of a small pocket lens. _Limpets_—those curious bonnet-shells, as they are termed in some places, which are found in the form of a flattened and inverted funnel, adhering closely to the flat rocks of the sea-shore in all the European seas—are more curious than they appear at a first glance, and have characteristics that well repay the labour of persevering observation. I should always place a few in an Aquarium.