. The elementary nervous system. Nervous system; Nervous system -- Coelenterata. 96 THE ELEMENTARY NERVOUS SYSTEM. longitudinal mesenteric muscles, by whose contraction the oral disc is depressed, may be called into action from almost any part of the outer surface of the sea-anemone. Paths of conduction must, therefore, exist between prac- tically every point of the exterior of the animal and the longitudinal muscles of the mesenteries, and for these paths of conduction the oral disc and the oesophagus are in no way essential. In fact, definite and circumscribed paths appear not to exist, but,
. The elementary nervous system. Nervous system; Nervous system -- Coelenterata. 96 THE ELEMENTARY NERVOUS SYSTEM. longitudinal mesenteric muscles, by whose contraction the oral disc is depressed, may be called into action from almost any part of the outer surface of the sea-anemone. Paths of conduction must, therefore, exist between prac- tically every point of the exterior of the animal and the longitudinal muscles of the mesenteries, and for these paths of conduction the oral disc and the oesophagus are in no way essential. In fact, definite and circumscribed paths appear not to exist, but, as the directions of the vari- ous incisions and cuts FIG. 29.—Metridium, seen from the oral shOW, it is nCCCSSary to aS~ pole, cut through vertically except for a email connecting bridge on the pedal edge SUniC, first, an immediate of the column. Stimulus applied at z. connection between the superficial ectoderm and the deep-lying musculature, and, next, a veritable nervous network by which all manner of irregular and complicated incisions can be circumvented so long as a bridge of the natural tissue remains as a means of connection between the part stimulated and the part responding. In other words, some of the most char- acteristic reactions of sea-anemones are to be ascribed to the fact that these animals possess a well-differentiated nerve-net (Parker, 1917 a}. The location of this nerve-net is by no means clearly determined. At first thought it might be expected to occur in the nervous sublayer described in the ectoderm and in the entoderm by the Hertwigs. But doubt has already been expressed as to the nervous character of this layer (Parker, 1912). In Metridium the so-called nervous sub-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Parker, George Howard, 1864-1955. Philadelphia, London J. B. Li
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