98 INVERTEBRATE ANIMALS. 



tal plates," perforated for the ducts of the reproductive organs; 

 the other set smaller, and each carrying a little " eye," hence 

 their name of " ocular plates." One of the genital plates is 

 also larger than the others, and carries a spongy mass which 

 is called the " madreporiform tubercle," and which protects the 

 entrance of the water-vascular or ambulacral system. The 

 whole of the test is covered with numerous tubercles of dif- 

 ferent sizes, which carry longer or shorter spines (Fig. 32). 

 The spines are jointed to the tubercles by a sort of " ball-and- 



FIG. 32. Cidaris papillata (after Gossc). 



socket " or " universal " joint, and they are completely under 

 the control of the animal, so as to be used both in locomotion 

 and apparently as defensive weapons. In most common species 

 the spines are short, but in many tropical forms they attain a 

 very great length. Besides the spines, the outer surface of 

 the test is furnished with curious little bodies called " pedi- 

 cellariae," which were long believed to be parasitic. They 

 consist of two or three blades mounted upon a flexible stalk 

 and constantly employed in snapping together like the beak 

 of a bird. They occur in many other Echinodermata^ and 

 their use is obscure. 



Locomotion is effected in the sea-urchins by a curious 

 system of contractile tubes which are known as the "ambu- 

 lacral tubes " or " tube-feet," and which are appendages of 

 the water-vascular system. The following is essentially the 

 arrangement of the whole aquiferous system. From the 

 madreporiform tubercle on the largest of the genital plates 



