VIII 



ECHINODERMATATHE CCELOMIG CAVITIES 



443 



In certain Clypeastroida (Echinodiscus Uforis, 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, Gonacls ; -2, constriction in a 

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

 system ; 6, tip of a Stewart's organ ; 7, forked radius of the masticatory framework ; S 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 perioesophageal sinuses from the general body cavity. 



