V. HOLOTHUROIDKA 



It passes backwards in the median dorsal interradius, forward in the left 

 ventral interradius, and then back in the right dorsal interradius to the 

 anus. It is held in position by mesenteries (fig. 309), and near the anus 

 by numerous muscular filaments. Into the terminal portion one or two 



FiG. 308. Anatomy of Caudina arenata (after Kingsley). a, anastomoses of 

 dorsal blood-vessel; b, branchial tree; d, dorsal blood-vessel;/, mesenterial filaments; 

 g, genital opening; /, alimentary canal; /, longitudinal muscles; t, mouth; <>, genital 

 duct; p, pharyngeal ring; r, gonads, cut away on right side; /, ampulke of tentacles; 

 v, ventral blood-vessel. 



branchial trees may empty. These are tubular sacs with small branched 

 outgrowths which are filled with water. They are respiratory, and are 

 periodically filled with fresh water. In many species 'L'ui'icrian organs' 

 occur; these are morphologically specially modified portions of the 

 branchial tree and are either connected with them or separately with the 



