THE DIGESTIVE TRACT OF CARASSIUS AURATUS

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Abstract
1. The digestive tube of Carassius auratus closely resembles that of Campostoma anonsalum (Rafinesque). 2. Study of the gross structure brought out the following characteristics: the posterior pharynx possesses pharyngeal teeth which bite on a dorsal horny pad; a dorsal diverticulum leading to the pneumatic duct extends from the short oesophagus; the oesophagus opens directly into an expanded part of the intestine, the intestinal bulb; a circular sphincter marks the boundary between oesophagus and intestinal bulb; the intestine proper does not show the extreme coiling of Campostoma but is considerably longer than that of Fundulus; it leads into the rectum. 3. The walls of the entire tract possess a tunica mucosa, tunica muscularis and tunica serosa. Absence of a muscularis mucosae prevents the determination of a distinct submucosa. 4. Epithelium of the pharynx is stratified with numerous mucous cells in the superficial layer. At the intestinal sphincter, there is an abrupt change to the simple columnar epithelium of the intestine. Stratified epithelium with mucous cells appears again in the rectum. 5. Columnar cells of the intestinal bulb and upper intestine show a band of darkly-staining cytoplasm between the nucleus and the luminal edge of the cell. In the lower half of the intestine, this region of each columnar cell is marked by a single vacuole which does not react to any of the stains used. 6. The tunica propria of the posterior pharynx and oesophagus is invaded with muscle fibers from the muscular layer. It forms a distinct region in the intestine. A stratum compactum, while distinguishable, is not a conspicuous feature. Mast cells are present, particularly in the intestine. 7. Striated muscle forms the tunica muscularis of posterior pharynx, oesophagus and intestinal bulb. It is gradually replaced by smooth muscle in the intestine proper. Between the oesophagus and intestinal bulb, the circular layer of muscle becomes enlarged to form the intestinal sphincter. 8. A serosa covers the entire tract beyond the most posterior part of the pharynx.