In spite of the foregoing assertions, I feel it my duty to caution the student against excess of labour. Let him ride his hobby cautiously, instead of seeking to enrol his name among the martyrs of science, of whom the noble Geoffry St. Hilaire, M. Sauvigny, and M. Strauss Dürckheim, are noted modern examples. Each member of this celebrated trio spent the latter part of his existence in physical repose, having become totally blind from intense study over the microscope. But setting aside the evils of excess, we must bear witness to the intense delight which this pursuit affords when followed with moderation. "'Tis sweet to muse upon the skill displayed (Infinite skill!) in all that _He_ has made: To trace in Nature's most minute design The signature and stamp of power divine. Contrivance intricate, expressed with ease, _Where unassisted sight no beauty sees_." As my aim is merely to give the reader a taste of the subject, and whet his appetite for its more extensive pursuit at other sources, I shall confine my remarks to a few of those creatures which are readily to be found in any well-stocked aquarium. The number of animalculæ and microscopic zoospores of plants, invisible to the naked eye, with which such a receptacle is filled, even when the water is clear as crystal, is truly marvellous. These animals mostly belong to the class _Infusoria_, so called from their being found to be invariably generated in any _infusion_, or solution of vegetable or animal matter, which has begun to decay. Now, the water in an aquarium which has been kept for any length of time necessarily becomes more or less charged with the effete matter of its inhabitants, which, if allowed to accumulate, would soon render the fluid poisonous to every living thing within it. This result is happily averted by the Infusoria, which feed upon the decaying substances in solution, while they themselves become in their turn the food of the larger animals. Indeed, they constitute almost the sole nutriment of many strong, muscular shell-fish, as pholas, mussel, cockle, &c.; and doubtless help to maintain the life of others, such as actiniæ, and even crabs, which, as is well known, live and grow without any other apparent means of sustenance. Thus the presence of Infusoria in the tank may be considered a sign of its healthy condition, although their increase to such an extent as to give a milky appearance to the water, is apt to endanger the well-being of the larger, though delicate creatures. The peculiar phenomenon alluded to arises from decaying matter, such as a dead worm or limpet, which should be sought after and removed with all possible speed. The whereabouts of such objectionable remains will be generally indicated by a dense cloud of Infusoria hovering over the spot. The milkiness, however, although it may look for the time unsightly, is ofttimes the saving of the aquarium 'stock.' When these tiny but industrious