AND STAR-FISH. 159 



are some called the " genital-plates." On one of 

 these is placed the " madreporiform tubercle," which 

 is perforated, and seems to act as a filter to allow 

 the water to pass through, but to prevent any solid 

 bodies entering with it. A canal proceeds, in- 

 ternally, from this tubercle to a central tube which 

 forms a ring round the gullet : thence proceed five 

 canals, radiating like the arms of a star-fish, and 

 passing along the interior until they meet at the top 

 of the shell. Each canal gives off a series of short 

 tubes in its course. Supposing the student had 

 emptied the sea-urchin of its internal soft parts, as 

 well as deprived it of its outer skin and spines, he 

 would then have seen, by holding the hollow and 

 empty shell up to the light, that it was constructed 

 of ten zones of plates, five of which were perforated 

 all the way up by rows of minute holes. Well, the 

 tubes given off by the internal canals which we 

 just mentioned pass through these holes, minute 

 though they be ; so minute, indeed, that the point of 

 the finest needle would more than fill one. In this 

 way, the tubes gain access to the outer world. At 

 their bases, on the other side the canal, is a little 

 water-bag connected with them. This has a mus- 

 cular power, and when it contracts the result is to 

 elongate the tube very much, and thus force it outside 

 the shell. These tubes are called " ambulacral " 

 feet, and each one is provided with a sucker at its 

 end, so that it can attach itself to a surface. A few 

 scores thus attached will warp or pull the body of 



