546 



IN VERTEBRA TE MORPHOLOO T. 



ing with the circumvisceral coeloni. A portion of one of the 

 ccelomic cavities at an early stage becomes cut off from the 

 rest of the ccelom and divided into five chambers whose walls 

 are formed of a dense fibrous membrane. This constitutes 



wp. 



FIG. 251. VERTICAL, SECTION THROUGH Antedon (combination of figures by 



LUDWIG and MARSHALL). 



ac = aboral canal of arm. 

 an aboral nerve. 

 Ax axial sinus. 

 Br = bracbial plates. 

 C cirri. 



cc = circumvisceral cavity. 

 CD = centrodorsal plate. 



co = central organ. 

 Do = dorsal organ. 



gr genital rachis. 

 / == intestine. 



ic = intervisceral cavity. 



M moutb. 



oc = oral cavity of arm. 



on = oral nerve-ring. 



B radial plates. 



rb radial lacunar vessel. 

 rn = radial epitbelial nerve. 

 ros rosette-plate. 

 rw = radial hydroccel-caual, 



sc = stone-canal. 



T oral tentacle. 



m = visceral septum. 

 wp = water-pore. 



the chambered organ (co), which in Antedon lies in a cavity 

 in the centrodorsal plate and is roofed over by the rosette- 

 plate, but in other forms simply rests upon the centrodorsal ; 

 communicating with it is the lower end of a somewhat club- 

 shaped structure termed the dorsal organ (Do), which pro- 

 jects orally parallel to the axial ccelomic cavity. 



The epithelium of the aboral ccelomic cavities of the arms 

 is differentiated here and there into peculiar organs the cili- 



