scavengers have completed their task of purification, they will cease to multiply, and mostly disappear, leaving the water clear as crystal. I believe it is the absence or deficient supply of Infusoria that sometimes so tantalizingly defeats the attempts of many persons to establish an aquarium. Pure deep-sea water, although never without them, often contains but very few, hence great caution is necessary not to overstock the tank filled with it, otherwise the animals will die rapidly, although the water itself appears beautifully transparent. Of Infusoria there are many species. They are nearly all, at one stage or other of their existence, extremely vivacious in their movements; so much so, indeed, that it becomes a matter of difficulty to observe them closely. Some have the power of darting about with astonishing velocity, others unceasingly gyrate, or waltz around with the grace of a Cellarius; while not a few content themselves by, slug-like, dragging their slow length along. The last are frequently startled from their propriety and aplomb by the rapid evolutions of their terpshicorean neighbours. Some, again, grasping hold of an object by one of their long filaments, revolve rapidly round it, whilst others spring, leap, and perform sundry feats of acrobatism that are unmatched in dexterity by any of the larger animals. I may here observe that the motions and general structure of many of the microscopic forms of vegetation, so much resemble those of some of the infusoria, that it has long puzzled naturalists to distinguish between them with any degree of certainty. The chief distinction appears to lie in the nature of their food. Those forms which are truly vegetable can live upon purely inorganic matter, while the animals require that which is organized. The plants also live entirely by the absorption of fluid through the exterior, while the animalculæ are capable of taking in solid particles into the interior of the body. Their mode of multiplication, and the metamorphoses they undergo, are much alike in both classes, being, during one stage of their existence, still and sometimes immovably fixed to stones, sea-weed, &c., and at another freely swimming about. Notwithstanding the similarities here stated, the appearance of certain of the species is as various as it is curious. One of the commonest species of the Infusoria (_Paramecium caudatum_) is shaped somewhat like a grain of rice, with a piece chipped out on one side, near the extremity of its body. It swims about with its unchipped extremity foremost, rotating as it goes. During the milky condition of the water (before alluded to), these creatures swarm to such a degree, that a single drop of the fluid, when placed under the microscope, appears filled with a dense cloud of dancing midges. Another (_Kerona silurus_) may be said to resemble a coffee-bean, with a host of _cilia_, or short bristles, on the flat side. These are used when swimming or running.