546 



INVEBTEBBATE MOBPROLOGT. 



ing witli the circumvisceral coelom. A portion of one of the 

 coelomic cavities at an early stage becomes cut off from the 

 rest of the coelom and divided into five chambers whose walls 

 are formed of a dense fibrous membrane. This constitutes 



fiG. 251.— Vertical Section through Antedqn 



LUDWIG and Marshall). 



(combination of figures by 



ac = aboral canal of arm. 

 an = aboral nerve. 

 Ax = axial sinus. 

 Br = brachial plates. 



= cirri. 



ce = circumvisceral cavity. 

 GD = centrodorsal plate. 



CO = central organ. 

 Do = dorsal organ. 



gr -— genital rachis. 



/ = intestine. 



ic = intervisceral cavity. 



Jf = moutb. 



oc = oral cavity of arm. 



on = oral nerve-ring. 



B = radial plates. 



rb = radial lacunar vessel. 

 rn = radial epithelial nerve. 

 ros = rosette-plate. 

 rw = radial hydrocoel-canal. 



se = stone-canal. 



T = oral tentacle. 



vs — 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- 



