THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 167 



which the heart is situated. To reveal this cavity more fully cut laterally along 

 the anterior face of the girdle keeping your instrument in contact with the girdle. 

 Portions of the girdle may be sliced away, but the heart must not be injured. 

 The pericardial cavity is thus revealed as a conical cavity lined by the parietal 

 pericardium and containing the heart. By gently lifting up the heart note 

 that it is attached only at its anterior and posterior ends. At these places 

 the pericardial lining is deflected from the walls of the pericardial cavity and 

 passes over the surface of the heart as a covering layer, the visceral pericardium, 

 which is indistinguishably fused with the heart wall. With the heart lifted 

 note that the posterior end of the heart is a fan-shaped chamber, the sinus 

 venosus, whose walls are continuous with the partition that forms the posterior 

 wall of the pericardial cavity. This partition is the transverse septum, whose 

 posterior face we have already seen. The septum is thus seen to be a partition 

 whose anterior wall is composed of the parietal pericardium and whose posterior 

 wall, of the parietal peritoneum. The wings of the sinus venosus are buried in 

 the transverse septum; they constitute large venous channels through which the 

 venous blood is returned to the heart. 



We may now explain the formation of the transverse septum. Since the 

 heart is situated in the ventral part of the body it is necessary in order that the 

 blood from the dorsal body wall may reach the heart that a bridge be formed 

 passing from the dorsal to the ventral side. In early embryonic stages a bridge 

 or cylinder of mesoderm develops on each side of the posterior end of the heart 

 connecting the splanchnic mesoderm surrounding the sinus venosus with the 

 somatic mesoderm of the dorsal body wall. In these bridges the main venous 

 channels pass from the dorsal body wall into the sinus venosus. Later, the 

 bridges enlarge and finally fuse with each other and with the body wall laterally, 

 forming a partition, the transverse septum, which thus cuts the heart off from 

 the remainder of the coelom. In elasmobranchs the fusion is not quite complete, 

 leaving an opening, the pericardio-peritoneal canal, dorsal to the sinus venosus, 

 extending from the pericardial cavity into the pleuroperitoneal cavity. This 

 opening will be seen at a later time. 



5. The mouth and pharyngeal cavities and the respiratory system. — Insert 

 one blade of a scissors into the left corner of the mouth and make a cut through 

 the angle of the jaws back across the ventral parts of the gill slits through the 

 pectoral girdle so that you emerge to the left side of the stomach. A flap is 

 thus formed which should be turned over to the right. A large cavity is revealed 

 which at its posterior end converges into the extremely short esophagus which 

 passes at once into the stomach. The esophagus may be slightly slit to aid in 

 opening the flap. 



The anterior part of the cavity inclosed by the jaws and gill arches is the 

 mouth or oral cavity. It is bounded in front by the upper and lower jaws, pro- 

 vided with teeth. The upper and lower jaws are the two halves of the first or 



