CHAPTER IV 



THE COELOM, DIGESTIVE SYSTEM, AND MESEN- 



TERIES 



1. The Coelom and Its Walls. 



Make an incision through the body wall of the skate just 

 anterior and to the left of the cloaca; run the incision 

 towards one side until the lateral wall of the cavity is 

 reached and then continue it forward along this margin 

 to the pectoral girdle. Do the same for the other side, 

 starting at the original incision. Leave the floor of the 

 body wall attached to the girdle. Be careful not to injure 

 the organs within the cavity. 



The body cavity of the vertebrates is the coelom, which 

 in its early stages is a continuous cavity extending the 

 entire length of the trunk region. In the adults of all 

 vertebrates this coelom is divided into at least two parts by 

 the formation of a transverse septum at the anterior end. 

 This septum separates the heart from the rest of the body 

 organs and will be located and studied later. The greater 

 part of the coelom which has just been opened is designated 

 as the pleuroperitoneal cavity. This cavity is partially 

 subdivided by certain mesenteries. It communicates with 

 the exterior through the abdominal pores. Probe into these 

 pores. 



The wall of the coelom is made up of these parts; the 

 skin, muscle, and a smooth shining inner membrane, the 

 pleuroperitoneal membrane. This membrane is divisible 

 into three categories ; the parietal peritoneum, which is that 



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