consisting of several cells in a state of union, the Synamœbæ, are able to perform all the functions of animal life—cohesion, sensation, motion, digestion, and reproduction; but, as the organism becomes more and more complex, these different functions are shared among several groups of cells. This differentiation proceeds steadily stage by stage, until at last different senses are located in different parts of the body, and we find animals possessing eyes, ears, noses, and mouths, one organ performing the function of sight, another that of hearing, and so on. All these organs of sense are but parts of the general nervous organisation of the body, which is _apparently_ absent in the Protista, but existing potentially in the protoplasmic substance, as it also does in every other substance in the universe. The ciliated multiple cell-organism, in course of time, becomes transformed into a hollow body, having a wall composed of a single layer of cells, and this again, by invagination, or folding of itself within itself, forms a double-walled cavity, or Gastrula, having an external opening like a mouth. These little animals, the Gastrœada, having an inner layer of cells (the endoderm), which carries on the nutritive and assimilative functions of the organism, and an outer layer (the ectoderm), which forms the general motor and sense-organ of the body, are the first animal organisms to possess a real sense-organ separate and distinct from other parts of the body. From this epidermal organ of sense are developed, as higher forms of animal life make their appearance, the nerve-cells and sense-cells which form the whole nervous system. In the fresh-water polyp, or Hydra, which is wanting in distinct organs of sense and nervous system, we find a remarkable sensitiveness to touch, warmth, and light, individual ectodermic neuro-muscular cells performing these functions, but a far greater sensibility being exhibited in the circle of fine prehensible tentacles surrounding the mouth than elsewhere. Here we have a marked attempt at localisation of sense-organs, and a manifestation of instinct, which makes the little animal shrink from the touch. From the Hydræ evolved the Medusæ, which, instead of being dependent entirely on neuro-muscular cells like the parent forms, developed minute sets of nerves and muscles, by the use of which they became enabled to swim about easily and at their own will and pleasure. We get in this little animal the first appearance of real nerve function, or conductibility of stimulus along the nervous fibre to a muscle which it causes to contract—a totally different function to the contraction of the whole body upon a stimulus being applied to it, as in the case of the Hydræ.