THE CLASSIFICATION OF ANIMALS 55 



spokes of a wheel round the hub. In Echinodermata there is 

 a complete digestive tube, surrounded by a spacious secondary 

 body cavity : of the primary body-cavity there are only traces. 

 Surrounding the mouth there is a ring-shaped tube containing 

 a watery fluid : this tube, which is in reality a special section 

 of the ccelom, is termed the water vascular system or 

 hydrocoele. It is perhaps the most characteristic organ of 



FIG. 25. Under view of common star-fish as an 

 example of Echinodermata; o, mouth; sp, 

 spines ; t, tube-feet. 



the Echinodermata. It gives off a number (usually five) of 

 branches which radiate outwards. In star-fish, brittle-stars, 

 and feather-stars these radiating tubes are supported by 

 arm-like outgrowths of the body ; but in the globular or disc- 

 like sea-urchins and in the sausage-shaped sea-cucumbers they 

 run back over the surface of the body like meridians of longi- 

 tude on a school globe. These tubes in turn give off to right 

 and left, paired outgrowths which project externally as tentacles, 

 and which form the sensory organs of the animal. They 



