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 ha\> 

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

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

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



i. The divisions and relations of the coelom. — The removal of the piastre 

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

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

 and the peritoneum is completely lacking in the ventral body wall of the turtk 

 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 th 

 peritoneum. Owing to this circumstance the parietal peritoneum can be easil; 

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

 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 shortb 

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

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

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

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

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

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

 while in fishes and Nccturus it formed the lining of a chamber surrounded b} 

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

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

 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. Cu 

 into the ventral wall of the pericardial sac, thus exposing the pericardial cavit] 

 and the contained heart. 



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

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

 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 thu: 

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

 peritoneum. 



The coelom of the turtle, like that of the fishes and Nccturus, consists of two parts, a smal 

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

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

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

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

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

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

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



