The Infusoria, the smallest of beings, apparently so insignificant, and for the most part invisible to the unaided eye, have high functions assigned to them in the economy of nature. They ‘are useful for devouring and assimilating the particles of decaying animal and vegetable matter from their incredible numbers, universal distribution, and insatiable voracity—they are the invisible scavengers for the salubrity of the atmosphere. They perform a still more important office in preventing the gradual diminution of the present amount of organic matter upon the earth. For, when this matter is dissolved or suspended in water in that state of comminution and decay, which immediately precedes its final decomposition into the elementary gases, and its consequent return from the organic to the inorganic world, these wakeful members of Nature’s invisible police are everywhere ready to arrest the fugitive organic particles, and turn them back into an ascending stream of animal life. Having converted the dead and decomposing matter into their own living tissues, they themselves become the food of larger Infusoria, as the Rotifera and numerous other small animals, which, in their turn, are devoured by larger animals as fishes, and thus a pabulum fit for the nourishment of the highest organized beings is brought back by a short route from the extremity of the realms of organized matter.’[23] SECTION III. HYDROZOA, ZOOPHYTES. ZOOPHYTES are animals of a much higher organization than the Protozoa, inasmuch as they are furnished with special organs of prehension, offence and defence, of attachment, and in many of locomotion. For the most part they consist of numerous individuals called Polypes, united in a community, and living together in intimate sympathy and combined action, so as to form one single compound animal. Zoophytes are divided into two groups, namely the Hydrozoa, whose type is the common fresh-water Hydra, and the Actinozoa, which are composite animals, including the reef-building corals, whose polypes are formed according to the type of the Actinia, or common Sea Anemone. The Hydrozoa consist of seven orders, the first of which are the Hydridæ, inhabitants of fresh water; the next constitute the oceanic Hydrozoa, some of which, though extremely varied in form, are connected by the most wonderful relations.