الفهرس 
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Abstract
In the present study live specimens of Siganus rivulatus, Mulloidichthys flavolineatus and Diplodus noct were collected from the Red Sea (Hurghada), Egypt. The digestive tract was studied by using light and scanning electron microscopes. The results can be summarized in the following items: 1. Measurements • It was observed that the mucosa in the anterior region of the oesophagus was thicker than in the middle and posterior regions of the studied fish species except in S. rivulatus. • Thickness of submucosal layer of oesophagus in the middle region was thicker than in the anterior and posterior regions of the studied fish species except in M. flavolineatus. • Thickness of oesophageal muscularis layer in the anterior region was thicker than in the middle and posterior regions of the studied fish species except in M. flavolineatus. • Mucosal layer of the stomach in the posterior region is thicker than other regions in fish species under investigation except in D. noct. • Thickness of submucosal layer of stomach in the anterior region was thicker than in the middle and posterior regions of the studied fish species except in M. flavolineatus. • Stomach muscularis layer thickness in the posterior region was thicker than in other regions of the fish species studied. • Intestinal mucosal thickness in posterior region was thicker than in the anterior and middle regions in studied fishes except in S. rivulatus. • Submucosal layer thickness of the intestine in the posterior region was thicker than in the middle and anterior regions of the studied fish species except in S. rivulatus. • Intestinal muscularis layer thickness in the posterior region was thicker than in other regions in all studied fish species. 2. Histology • Microscopic examination of transverse sections of the digestive tract wall of the studied fish species showed that it composed of mucosa, submucosa, muscularis and serosa. • Oesophageal mucosal epithelium composed of columnar cells in S. rivulatus but in M. flavolineatus it changed gradually from cubic to columnar cells, Oesophageal mucosa of D. noct was lined by stratified squamous filled with mucous cells, it changed to simple columnar epithelium at the middle and posterior regions. • Muscularis of the digestive tract in the studied regions of studied fish species was composed of two layers except in anterior and middle regions of stomach of M. flavolineatus and in posterior region of oesophagus of D. noct. • Stomach mucosa of the studied fish species was lined by simple columnar epithelium and the gastric glands were present. • Intestinal mucosa of the studied fish species was lined by simple columnar epithelium with goblet cells that increased in posterior region. 3. Morphology of the mouth cavity In the present study the oral cavity opens through a small mouth opening in S. rivulatus and D. noct and through a transverse mouth opening in M. flavolineatus. Upper and lower velums were observed in S. rivulatus and D. noct but absent in M. flavolineatus. 4. Scanning electron microscope observations The results of the present study revealed that S. rivulatus has three kinds of teeth; tri-cusped, bi-cusped, papilliform but M. flavolineatus has four kinds of teeth; curved – blunt, cardiform monocuspid sharp crowns, molariform and papilliform, and D. noct has four kinds of teeth; broad incisor-like, molariform, curved papilliform and pointed-cannine like teeth. Three types of taste buds (type I, II and III) were recorded in the oropharyngeal cavity of M. flavolineatus and two types of taste buds (type I and III) were recorded in D. noct while taste buds of type -I were recorded in S. rivulatus. Mucous is elaborated by mucous cells which observed in the mouth cavity of fish species under investigation, in oesophagus of M. flavolineatus and in the digestive tract of D. noct. The present study revealed presence of microridges covered the surface cells in the oral cavity and oesophagus of M. flavolineatus and in the mouth cavity of S. rivulatus and D. noct. Gastric glands were observed in the anterior and middle regions of the stomach of M. flavolineatus and in the three regions of the stomach of D. noct and S. rivulatus. The complex nature of mucosal folds was observed throughout the digestive tract (oesophagus, stomach and intestine) of the fish species under investigation.