172 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



E. THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS OF THE TURTLE 



Obtain a specimen and place in a dissecting pan. Specimens which have 

 not been injected should be employed. Remove the plastron. This is done by 

 sawing through the bridges on each side, lifting up the plastron and separating 

 it with a scalpel from the surrounding skin and underlying membrane. 



i. The divisions and relations of the coelom. The removal of the plastron 

 exposes a membrane, the parietal peritoneum, which covers and conceals the 

 viscera. Note that the muscle layer which is normally present between the skin 

 and the peritoneum is completely lacking in the ventral body wall of the turtle, 

 owing to the presence of the plastron. The ventral body wall in turtles there- 

 fore consists of but two layers, the skin with its contained exoskeleton, and the 

 peritoneum. Owing to this circumstance the parietal peritoneum can be easily 

 separated from the inside of the body wall, a procedure which is difficult or 

 impossible in other vertebrates. Note, however, the usual muscles in connec- 

 tion with the girdles and limbs. 



In the median line in the anterior part of the parietal peritoneum shortly 

 posterior to the pectoral girdle is situated a triangular membranous sac, the 

 pericardial sac, which incloses the heart. It will be noticed that the heart is 

 much more posterior in position than is the case in the fishes and Necturus; in 

 fact, there has occurred a posterior descent of the heart (and of other viscera as 

 well). The membranous sac covering the heart is, as in the dogfish, the parietal 

 pericardium. Here it takes the form of an isolated sac, the pericardial sac, 

 while in fishes and Necturus it formed the lining of a chamber surrounded by 

 the body wall. The space between the pericardial sac and the heart is the 

 pericardial cavity, a portion of the coelom. The ventral face of the pericardial 

 sac rests in the natural position against the internal surface of the plastron, 

 while its dorsal face is fused, as we shall see, to the parietal peritoneum. Cut 

 into the ventral wall of the pericardial sac, thus exposing the pericardial cavity 

 and the contained heart. 



Two conspicuous veins, the ventral abdominal veins, run longitudinally in the 

 parietal peritoneum between the pericardial sac and the pelvic girdle. Cut 

 through the peritoneum halfway between the heart and pelvic girdle by a trans- 

 verse cut which severs both of the abdominal veins. The large cavity thus 

 exposed is the pleuro peritoneal cavity, whose walls are lined by the parietal 

 peritoneum. 



The coelom of the turtle, like that of the fishes and Necturus, consists of two parts, a small 

 pericardial cavity and a much larger pleuroperitoneal cavity. We note, however, that whereas 

 in the lower forms the pericardial cavity is anterior to the pleuroperitoneal cavity and separated 

 from the latter by the transverse septum, in the turtle the pericardial cavity is ventral to the 

 pleuroperitoneal cavity, and the transverse septum seems to have disappeared. We may 

 explain this change as follows. (See also Fig. 45, p. 160.) In its posterior descent the heart must 

 necessarily carry with it the transverse septum and the parietal pericardium. The latter in 

 order to move posteriorly must separate from the body wall to which it is attached in lower 



