HOLOTHUROIDEA. 



261 



with two intestinal vessels, one of whicli lies on the mesenterial 

 side of the intestine close to the insertion of the mesentery, and 

 the other on the op- 

 posite, non- mesente- 

 rial side (Fig. 181). 



In many Holothurians 

 (most Aspidochirotae, 

 sonic Dendrochirotac 

 and Molpadiidae) the 

 dorsal gut vessel (mesen- 

 terial) becomes detached 

 in part of its course i'roi i 1 

 the intestinal wall (Fig. 

 IS,'}), but remains con- 

 nected with the blood 

 lacunae in t he intest inal 

 wall by a plexus of ves- 

 sels passing across the 

 body-cavity. This plexus 

 is the rete mirabile. The 

 dorsal vessel and rete 

 mirabile lie on the same 

 side of the mesentery. 

 It often forms a kind of 

 web \\hicli loosely in- 

 vests the terminal bran 

 ches of the left respira- 

 tory tree. These vessels 

 are connected with 

 lacunae in the walls of 

 the neighbouring organs. 

 The intestinal vessels 

 are connected with 

 lacunae in the inner con- 

 nective tissue coat of 

 the alimentary canal ; 

 the circular vessel sup- 

 plies the tentacular 

 canals, the stone canal 

 and the polian vesicle, FIG. 183.-Holothuria tubulosa opened longitudinally (from 

 Claus, after M. Edwards). mouth in the midst of the 

 tentacles T ; D alimentary canal ; Sc stone -canal ; P 



polian vesicle ; Rg circular vessel of the water- vascular 

 system ; Ov ovary ; Ag radial water-vascular canal ; M 

 longitudinal muscles ; Of intestinal vessel ; Cl cloaca ; Wl 

 respiratory trees. 



the oesophagus and 

 often the gonad ; the 

 radial vessels give off 

 branches to the tube- 

 feet and papillae ; and 



lastly the lacunae in the walls of the gonad may be supplied from the 

 dorsal (mesenterial) intestinal vessel, or from the circular vessel. 



The vascular system is composed of tissue similar to that 

 found in Asteroids and Echinoids, but containing better defined 



