Fig. 383. —Organisation of an Aspidochirotan Holotliurian. In the dorsal interradius, the 

 body wall is cut through on the left, near the dorsal mesentery, and is spread out (after Ludwig, in 

 Leuckart's Tafelwerlc). 1, Genital aperture ; 2, radial plates ; 3, interradial plates of the calcareous 

 ring ; 4, genital duct ; 5, dorsal or anterior mesentery of the intestine ; 6, stone canals with their 

 inner madreporites ; V, dorsal intestinal vessel ; 8, gonads ; 9, anterior section of the alimejitary 

 canal ; 10, ventral intestinal vessel ; 11, posterior section of intestine ; 12, longitudinal muscles ; 13, 

 posterior edge of the dorsal mesentery ; 14, right branchial tree (water lung) ; 15, circular muscula- 

 ture of the body wall ; 16, longitudinal muscles ; 17, cut edge of the body wall ; 18, longitudinal 

 muscles ; 19, partly muscular filaments running from the wall of the cloaca to the body wall ; 20, cloa- 

 cal aperture ; 21, cloaca ; 22, Cuvierian organs ; 23, left branchial tree ; 24 and 25, longitudinal muscles ; 

 26, middle section of the alimentary canal ; 27, vascular anastomosis ; 28, fore-gut ; 29, Polian 

 vesicle ; 30, blood vascular ring ; 31, water vascular ring ; 32, commencement of the radial vessel. 



