Coetenterate 



GastrovascuIoT 

 cavity 



Flatwomj 

 Lumen 



Roundworm 



Annetid 



PseucJocoet 



CoeTom' 



I Mesoderm ^ Endoderm 



The relationship of body cavities to germ cell layers 



Intestinal epithelium" 



Typhlosole - 

 lumen- 



Peritoneum - 



.^.^ssSSSi 



Dorsal blood vessel 



Chloragogue cells 

 Coelom 



Muscle 



Epidermis 



Nephridium (section) 

 Circular muscle 

 Longitudinal muscle 

 Ventral nerve cord 

 Subneural blood vesse 



NepHridiopore 

 Tubule of nephridium 



Nephrostome 



Lateral neural blood vessel 

 Ventral blood vessel 



Figure 92. Top, cross sections of a coelenterate, flatworm, roundworm, and annelid, designed 

 to show the relationship of body cavities to the germ layers. Bottom, cross section of an 

 earthworm, which illustrates the advances in complexity of structure, correlated with the appear- 

 ance of a coelom and the development of systems of organs. Left side of drawing shows sec- 

 tioned parts of nephridium as they actually appear, and right side shows an earthworm nephridium 

 as it appears in a dissection. Rarely does a cross section show all four pairs of setae. 



174 



