THE AXIAL GRADIENTS IN HYDROZOA

Abstract

1. A study of the susceptibility of three species of hydra, H. viridissima (viridis), H. vulgaris (grisea) and H. oligactis (fusca) to various agents shows that a gradient in susceptibility exists along the apico-basal axis of the body and in each tentacle. 2. This susceptibility gradient is primarily a basipetal gradient, i. e., the susceptibility decreases from the apical region basipetally and after the tentacles arise, in each tentacle from the tip to the base. This primary gradient appears in the earlier bud stages, in quiescent attached individuals of H. viridissima and H. vulgaris at all stages and in young quiescent individuals of H. oligactis. 3. This primary gradient is modified in various ways by regional differences in functional activity and particularly by the special activities of the stalk, a more or less specialized organ which develops at a relatively late stage from the basal body region. 4. Regional differences in the contractile activity of the muscular apparatus produce the most conspicuous modifications of the susceptibility gradient. Such modifications can be produced by detachment of the animals, which leads to special activity of the stalk, by the general excitatory or special irritant action of certain agents, such as the dyes used, and by certain combinations of excitation and paralysis with anesthetics. 5. Alterations of the primary susceptibility relations are produced, not only by regional differences in muscular activity, but by regional differences in digestive activity. 6. The very direct and marked effect upon susceptibility of regional and local differences in muscular and other functional activity results from the simple and diffuse organization of hydra. The muscle, for example, is a part of the body cell and the whole cell shares in the excitation connected with muscular contraction, and the facts indicate that most or all local excitations spread or irradiate to a greater or less degree to adjoining regions. 7. In stages after the development of motor activity susceptibility decreases in general with advancing physiological age, but the bud in stages preceding the development of motor activity is less susceptible than later motile stages. 8. The differences in the susceptibility relations of the three species under various conditions suggest that of the three H. oligactis possesses, as its structural and functional characteristics indicate, the highest degree of regional specialization. 9. The facts confirm previous work in that they indicate the existence of a relation of some sort between susceptibility and metabolic rate. Within the ranges of concentration of the agents used, susceptibility evidently varies in general directly with metabolic rate, or with the rate of energy-liberating metabolism. If such a relation exists, the axial gradient is a gradient in physiological condition, of which rate of fundamental metabolic reactions is one feature.