its bell eight clusters of sense-organs. Each cluster contains an ocellus, two sensory pits that are probably concerned with the chemical sense, and a sense-club which may be a pressure organ. The sensory portions of all these organs are modified ectoderm and from these portions nervefibers pass out as radiating bundles to the ectoderm of the subumbrellar surface. Here they merge into a nervous net which overlies the ectodermic musculature as in the sea-anemones. 
Fig. 4. Aurelia, subumbrellar surface; s, cluster of sense-organs. This musculature forms a circular sheet concentrically disposed with reference to the symmetry of the jellyfish. When the bell of an Aurelia is pulsing, the movement is carried out by the more or less general contraction of this circular band of muscle, which is brought back to its original position on relaxation by the elasticity of the gelatinous mass of the bell. The locomotor muscle, then, is a gigantic sphincter that works against an elastic resistance.
The significance of the various parts of the neuromuscular mechanism in such an animal as Aurelia can be determined by experiment. If the eight sense-bodies are removed, the animal will no longer pulse spontaneously, though its muscles may be made to contract by direct stimulation. If all but one sense-body are removed, the bell will pulse with regularity and by artificially stimulating the single remaining body a wave of muscular contraction can be sent over it. It is therefore evident that the sense-bodies act like extremely delicate triggers and thus touch off the contractile mechanism. In this respect, then, the jellyfish is more highly developed than the sea-anemone, for the latter possesses no such specialized and delicate receptors.
The wave of contraction that passes over a bell when one of its sense-bodies is stimulated, may be either a purely muscular phenomenon or may be the result of nervous transmission through the nervous net whereby one region after another of the musculature is brought into action. The fact that this wave is not checked when the bell is cut even in a most irregular way provided the subumbrellar epithelium is still continuous, favors the nervous rather than the muscular interpretation. But stronger evidence on the nervous side than this has come from an entirely different direction. Mayer (1906) has shown that the subumbrellar epithelium of Cassiopea after removal will readily regen-
