60 The Under-Water World 



an urchin is in constant motion, for, 

 besides spines and pincers, there is a 

 regular series of " water feet." A hollow 

 ring running round the inside of the 

 animal gives off five canals set with 

 numerous tubes which can be filled with 

 water, causing them to expand and 

 stretch forwards. One of these tubes or 

 r< feet " having taken a firm grip of the 

 ground with its circular tip, presses 

 backwards when another " foot " comes 

 into play. A complex system of canals 

 lined with innumerable hairs perpetually 

 lashes the water and supplies a steady 

 stream to the system of " tube feet." 

 Once these hairs cease to beat, the 

 animal comes to rest. A striking in- 

 ternal feature of the urchin is the mouth 

 with its fine chisel-edged teeth situated 

 on the under surface of the animal. 



Urchins reproduce by laying eggs, 

 our commonest form depositing about 

 20,000,000 in a year, and the egg masses 



