VIII 



ECHINODERMATA THE CCELOMIC CAVITIES 



443 



In certain Clypeaitroida (Echinodiscus biforis, Peronella orbicularis) small inter- 

 radial, thin -walled, vesicular outgrowths of the lantern membrane, on the base of 

 the lantern itself, have been called Stewart's organs. Two of these occur in each 



FIG. 370. Viscera of Asthenosoma (after F. and P. Sarasin). 1, Gonads ; 2, constriction in a 

 Stewart's organ ; 3, Stewart's organ ; 4, muscle lamellae ; 5, radial canal of the water vascular 

 system ; 6, tip of a Stewart's organ ; 7, forked radius of the masticatory framework ; 8 and 

 '.'. upper and lower coils of the intestine ; 10, Polian vesicle ; 11, intestine. 



interradius, but are generally wanting in that interradius in which the intestine in 



ascending lies upon the lantern. 



3. Ophiuroidea. Two circular membranes, one above the other, 

 found round the oesophagus, and traversing the body cavity, connect 

 the oesophagus with the oral skeleton. They thus cut off two very 

 small pericesophageal sinuses from the general body cavity. 



