In the simpler multicellular organisms, which develop by cell division and multiplication from a single cell, the cells differ from the original type and from each other in position, structure, and function. In the course of growth the organism originally spherical in shape becomes modified by irregular growth of cells, producing folds and prominences. Cells are crowded out of shape; some lie at the base of a depression protected from stimulation; others occupy positions which make them especially liable to be acted upon by such stimuli. In the course of these modifications some of the cells become especially adapted for receiving impressions, others for conducting or transmitting these impressions to various parts of the organism, others for producing movements of the organism. It is with the first type of cell that we are concerned, the receptor mechanisms. They are in the simpler organisms, adapted to receive two sorts of stimulation, mechanical and chemical. In fact, through the whole series of multicellular organisms such reactions to mechanical and chemical stimuli have been noted more or less definitely, although special sense organ structures have in many cases not been discovered. This is especially true for the reactions to chemical substances. It is customary to speak of the “chemical sense,” to signify these responses to chemical substances, without any attempt to differentiate between smell and taste. Obviously, in the case of organisms which live in a fluid environment, this chemical sense might be called taste, since it would correspond in a way to that sense in man, for which the adequate stimulus is a fluid. But since it is a “distance receptor,” in that objects at a distance can produce responses, probably by diffusion of substances in the fluid, it might also be looked upon as more nearly resembling the smell sense. In most cases structure offers no help in settling the matter. In the medusa, or jellyfish, one of the earliest forms in which a nervous system and sense organs are found, the tentacles are especially sensitive to chemical stimuli, much less so to mechanical stimuli. To the former they respond by shortening and twisting themselves about the object. As for sense organs in these parts, there are small club-shaped papillæ in the neighborhood of the tentacles, differing somewhat in character in the different species. These papillæ contain a narrow canal lined with thick cylindrical cells. As far as both structure and function are concerned, they may be considered either as taste or smell organs.