There is an essential structure which we endeavour to assign to all the animals of each phylum, and which is different from the structure of the animals belonging to all other phyla. The Protozoa, which for the present we regard as animals, are organisms the bodies of which consist of single cells. These cells may become aggregated into colonies, but they may as well exist apart from each other. They may be enclosed in limy, siliceous, or cellulose skeletons or shells, or they may possess limy or siliceous spicules in their tissues--these parts are non-essential, and the schematic Protozoan is a cell containing a single nucleus, and capable of independent existence. The Porifera, and all the other phyla, include organisms the bodies of which are made up of aggregates of cells. In the Porifera the cells, which are specially modified in structure, are arranged to form the internal walls of a “sponge-work” the cavities of which open to the outside by series of pores through which water is circulated. The bodies of the Cœlenterates are typically sacs formed by a double wall of cells--endoderm and ectoderm. This sac opens to the exterior by a single opening, or mouth, surrounded by a circlet of tentacles, and its cavity is the only one contained in the body of the animal. The Platyhelminth worms are animals the bodies of which are also composed of ectodermal and endodermal tissues, between which is intercalated another mesodermal tissue. They have a single digestive sac or alimentary canal opening to the exterior by means of a mouth only; and they all possess a complex, hermaphrodite, reproductive apparatus. In all the other phyla there are also three principal layers or kinds of tissue, but in addition to the cavity of the alimentary canal there is also a body cavity, or cœlom, which is contained in the mesodermal tissues. The Echinoderms are such cœlomate animals, but the alimentary canal now opens to the exterior by means of both mouth and anus; there are separate systems of vessels through which water and blood circulate; the blood-vascular system of vessels is closed to the exterior, the water-vascular system being open; and the integument is armed by means of calcareous spines or plates. The Annelids are animals with cylindrically shaped bodies, segmented so as to form numerous joints. Each segment bears spines or hairs or appendages of some sort, and also contains a separate nerve-centre. The alimentary canal opens externally by a mouth and anus, and there is a spacious body cavity. The Molluscs are unsegmented animals. The dorsal part of their bodies contains the viscera, and is protected by a shell; while the ventral part is modified for the purpose of locomotion. A fold of integument hangs down all round the body and encloses a cavity in which the gills are contained. The Arthropods are segmented animals. The body is armed by a calcareous carapace or shell which forms the exo-skeleton. Each