106 INVERTEBRATE ANIMALS. 
“ambulacral tube-feet”). These zones are therefore called the 
‘‘ambulacral areas.” Besides these main rows of plates which 
collectively make up the greater part of the test, there are other 
plates placed in the leathery skin round the mouth and vent. 
The most important of these form a kind of disc, which is 
placed at the summit of the shell. This disc (fig. 41, 3) is com- 
posed of two sets of plates—one called the “ genital plates,” per- 
forated 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 different sizes, which 
carry longer or shorter spines (fig. 42). The spines are jointed 
i KL 
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Fig. 42.—Cidaris papillata (after Gosse). 
to the tubercle by a sort of “ ball-and-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 of our British 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 “ pedicellariz,” which were 
long believed to be parasitic. They consist of two or three 
blades mounted upon a flexible stalk and constantly employed 
