HOLOTHURIANS OR SEA-SLUGS. 



43 



radial branches, makes up one of the most characteristic struc- 

 tures of Echinoderm organisation— the system of water-vessels. 

 In the Starfish it communicates directly with the exterior by a 

 canal which opens on the upper surface of the body, but which 

 in the Sea-slug hangs freely in the body cavity and draws its 

 water thence. On the circular vessels and its trunks there are 

 swellings or sacs with contractile walls, and it is by the action 

 of these that the water is propelled through the system ; when 

 it is driven into the tentacles or the suckers these parts are 

 forthwith erected. 



rn. 



Fig. 2. — Internal Structuee of a Holothdeian. o, Mouth; /, Intestine; 

 d, Cloaca; a, Vent; c, Branched stone-canal ; p, Polian vesicle ; rr. Lungs; 

 »•', their point of insertion into the Cloaca ; ?«, Longitudinal muscle. (From 

 Gegenbaur, ' Comparative Anatomy,' Bell's translation, p. 215.) 



The intestine is looped and opens by a vent at the end of the 

 body opposite to the mouth ; its walls are exceedingly delicate, 

 and owing to the fact that the contents are often sandy and 

 gravelly, or composed of calcareous debris, they are frequently 



