iv THE INTERNAL STRUCTURE 79 



teries and the similar sheets of membrane supporting the 

 ovaries, oviducts, and testes are double, as would naturally 

 be the case if these organs were pushed in in the way men- 

 tioned. The peritoneum passes down from the body wall, 

 covers these organs, then passes back to the body wall again, 

 the two sheets of membrane coming close together except 

 where they are separated by the organ they surround. The 

 arteries and veins supplying the organs generally run be- 

 tween the two layers of the supporting membranes. The 

 portion of the peritoneum surrounding the alimentary canal 

 and its appendages is called the visceral layer; the part 

 applied to the body wall, the parietal layer. For the most 

 part the parietal layer is grown fast to the body muscles, 

 but on the dorsal side of the body it is separated from the 

 wall, forming a large lymph space, the cistema magna, or 

 subvertebral lymph sinus. The kidneys lie in this space ; 

 hence they are covered with peritoneum only on the ventral 

 side, and not completely invested by it like the other 

 viscera. The membrane previously mentioned, which extends 

 between the ventral body wall and the pericardium and 

 liver, is a portion of the peritoneum, forming a.sort of ventral 

 mesentery ; it is reflected upon the ventral body wall on 

 the one hand and spread out over the pericardium and liver 

 on the other. 



The ccelom is filled with a transparent fluid, the ccelomic 

 ox peritoneal fluid, which is essentially the same as the lymph 

 found in other portions of the body. Owing to the fluid in 

 which they lie and the smoothness of their peritoneal coat- 

 ing, the organs in the body cavity are enabled to glide over 

 each other with little friction. 



Organs outside of the Body Cavity. — Above the ccelom 

 there is a second cavity surrounded by the bones of the ver- 

 tebral column and skull and containing the central nervous 



