474 



ZOOLOGY 



the inner their arrangement is longitudinal. The circular layer is 

 continuous, or, more usually, is interrupted opposite the intervals 

 between the segments. The longitudinal layer is disposed in four 

 bands in the PolychaBta, two dorso-lateral and two ventro-lateral. 

 In the Oligochfeta it is divided by the setigerous sacs which pass 

 through it. 



The peritoneal or ccelomic epithelium consists of a single layer of 

 cells. These are usually non-ciliated, but are ciliated in the 



Fig. 373.— Terebella. (After Quatretages.1 



Aphroditea, Glyeera, and some others, the movement of the cilia 

 bringing about an active circulation of the cmlomie or perivisceral 

 fluid in the coelome. 



The body-cavity or coelome, a wide space intervening between 

 the wall of the body on the one hand and that of the enteric 

 canal on the other, is divided in many Chsetopoda by a series of 

 transverse septa into a series of chambers corresponding to the 



