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THE BOOK OF NATURE STUDY 



green spines tipped with purple. The urchin has, however, the 

 habit of concealing itself by attaching fragments of weed, etc., to 

 its spines, so that it is very liable to be overlooked. A specimen 

 cleaned from these adventitious particles will show the five 



double rows of slender 

 tube-feet, which protrude 

 among the spines, and 

 which are the organs of 

 locomotion. These tube- 

 feet are filled with sea- 

 water, and communicate 

 with an elaborate system 

 of canals, forming together 

 the water- vascular system, 

 into which seawater enters 

 by a perforated plate, 

 called the madreporite, 

 placed near the anus on 

 the upper surface of the 

 test. The mouth is placed 

 in the middle of the under 

 surface. Within its cavity is contained a curious system of hard 

 bars, or ossicles, which carry five teeth. These five teeth can be 

 protruded through the mouth opening and enable the urchin 

 to obtain its food. The structure which bears these teeth was 

 first described by Aristotle in the common urchin, and is still 

 called Aristotle's lantern. 



The purple-tipped urchin is the only urchin with which much 

 success is likely to be obtained in the aquarium, but there is one 

 other form which may be kept temporarily to show its special 

 adaptations. This is Echinocardium cor datum, or the heart- 

 urchin, whose fragile tests, scrubbed clean by the waves, are 

 nearly always common on sandy shores after storms. At the 

 same period one may sometimes find specimens still furnished 

 with their golden spines, but for the living animal it is necessary 

 to dig in the sand close to low- tide mark. Here the spade will 

 turn up many of all sizes, all clothed with silky golden spines. 

 An attempt should be made to keep some of the smaller forms 



FlG. 75. The common sea-urchin (Echinus escu- 

 lenttis). The spines have been partially re- 

 moved to show the structure of the test, a, an 

 ambulacral area, i.e. one where the tube-feet 

 emerge ; z, an area without tube-feet (inter- 

 ambulacral). 



