Page:Popular Science Monthly Volume 75.djvu/141

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HE second step in the development of the neuromuscular mechanism is represented by the receptor-effector system, a condition fairly realized in such cœlenterates as the sea-anemones and the jellyfishes and probably recurring in the digestive tubes of the higher metazoans. As an introductory example we may turn to the sea-anemones.

Most sea-anemones (Fig. 1.) are cylindrical animals attached to some firm object by their aboral disks and carrying on their oral disks a ring of tentacles surrounding the mouth. This aperture leads inward through a short gullet to a large, somewhat divided, digestive cavity, the gastrovascular space, which extends throughout the whole interior of the animal even to the tips of its tentacles and is the only cavity within the sea-anemone. The body of the animal is made up of walls of extreme thinness; these walls consist of two layers of cells, an outer one next the sea water, the ectoderm, and an inner one next the gastrovascular space, the entoderm. These two layers are separated by a tough, non-cellular sheet, the supporting lamella.

Unlike sponges, sea-anemones are very responsive to changes in their environment. If a fully expanded Metridium is disturbed by mechanical agitation, it will quickly retract its oral disk, discharge through its mouth the water contained in its gastrovascular cavity, and