. Principles of modern biology. Biology. PARENCHYMA (MESODERM) which illustrates the advantages of such a system. Annelida, like higher forms gener- ally, have developed a sharply defined coelo- mic cavity, which intervenes between the wall of the digestive tract and the body wall (Fig. 16-10). Moreover, the gut wall has developed a separate musculature, so that the digestive movements of the animal are independent of its external movements. The gut muscula- ture takes care of churning, mixing, and moving the food mass in the digestive tract, while simultaneously the muscles of the body wall a

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. Principles of modern biology. Biology. PARENCHYMA (MESODERM) which illustrates the advantages of such a system. Annelida, like higher forms gener- ally, have developed a sharply defined coelo- mic cavity, which intervenes between the wall of the digestive tract and the body wall (Fig. 16-10). Moreover, the gut wall has developed a separate musculature, so that the digestive movements of the animal are independent of its external movements. The gut muscula- ture takes care of churning, mixing, and moving the food mass in the digestive tract, while simultaneously the muscles of the body wall are executing the various external re- sponses of the animal. Tubular enterons are variously modified in different higher animals. In the earth- worm, the pharynx lies directly behind the mouth and displays a relatively thick muscu- lar wall (Fig. 16-9). The pharynx acts as a sort of suction pump that expands and con- tracts, sucking in small masses of moist earth as the worm burrows into the soil. The ingested earth, which contains considerable rotting plant material, mixed with the hard soil particles, then passes through the straight cylindrical esophagus (segments 6 to 14) to a. INTESTINE GIZZARD CROP CALCIFEROUS ESOPHAGUS GLANDS Fig. 16-9. Tubular digestive tract of the earthworm (diagrammatic). Note the differentiation of the suc- cessive parts.. 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 work.. Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston