THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 171 



The pericardial cavity is a conical canity lined by the parietal pericardium. 

 On gently raising the heart the posterior wall of the cavity is seen to be formed 

 by the transverse septum. The transverse septum is pierced by two veins 

 (hepatic sinuses) which extend forward and enter the sinus venosus, the most 

 dorsal chamber of the heart. 



4. The oral cavity and the pharynx. — Open the mouth and cut through the 

 angle of the jaws on each side so that the jaws can be spread widely. Carry your 

 cuts back to the gill arches. The cavity thus exposed consists of an anterior 

 oral cavity and a posterior pharynx. 



The oral cavity is bounded externally by the well-developed lips. Internal 

 to the lips are the small conical teeth. There are two rows of teeth on the roof 

 of the mouth, the posterior row being the longer. External to the last teeth of 

 the posterior row on each side is a slit, one of the posterior nares or internal 

 openings of the nasal passages. Probe into one of the anterior nares and note 

 emergence of the probe through the posterior naris. The floor of the mouth 

 cavity bears a single row of teeth which on closing the mouth will be found to fit 

 between the two rows on the roof. Posterior to the teeth is the tongue supported 

 by the strongly developed hyoid arch which should be felt within the tongue. 



The walls of the pharynx are pierced by two pairs of gill slits. Probe through 

 them and note emergence of the probe between the external gills. Note the 

 cartilaginous gill arches supporting the bars (visceral arches) between and on 

 each side of the gill slits. The walls of the gill slits are the visceral pouches. 

 Unlike the dogfish they bear no gills, the gills being external. The pharyngeal 

 cavity narrows posteriorly into a tube, the esophagus. By passing a probe into 

 the esophagus determine that it extends dorsal to the pericardial cavity and is 

 continuous with the stomach. 



5. The larynx and the lungs.— In the floor of the pharynx midway between 

 the second gills slits will be found a short slit, the glottis. The walls of the glottis, 

 as should be determined by feeling them with a fine forceps, are stiffened by a 

 pair of delicate cartilages, the arytenoid cartilages. These probably represent 

 reduced gill arches. They are the first of the laryngeal cartilages to appear in 

 the phylogenetic series. The small cavity into which the glottis leads and which 

 is inclosed between the two arytenoid cartilages is the larynx. Cut across the 

 gill slits of the left side so that the pharyngeal cavity can be opened more widely. 

 Slit the glottis posteriorly. The larynx is thus seen to lead into a narrow flattened 

 passage, the trachea. The posterior end of this is widened and receives two 

 openings. Probe into each with a slender probe and note emergence of the probe 

 into a lung. The trachea is thus seen to communicate with the lungs. The air 

 passage in primitive air-breathing vertebrates takes the following course: 

 anterior nares, nasal cavities, posterior nares, oral cavity, pharyngeal cavity, 

 glottis, larynx, trachea, lungs. The lungs have already been noted. Slit open 

 one of them and note the smooth interior, not provided with air sacs. 



