FOSSIL STAR-FISHES AND SEA-URCHINS. 145 



removed, and hold it up so that the light is seen 

 through it. Besides the large apertures at the top 

 and the bottom (anus and mouth), you perceive rows 

 of minute punctures radiating, from the summit to 

 the base. These punctures are called " ambulacral 

 pores," and the plates (of which there are five rows) 

 in which the pores occur are termed "ambulacral 

 plates," for a reason that will shortly be seen. In 

 addition to these, there is a plate specially perforated, 



Fig. 119. Echinus esculenta. On left-hand side is a fragment of test denuded of 

 spines, and showing how the plates are mosaicked. 



called the " madreporiform tubercle " (for it is porous 

 and spongy, like the common Madrepore coral), 

 and its office seems to be to admit the sea- water as 

 through a filter. From this a sort of canal proceeds 

 internally to a tube which surrounds the gullet at the 

 base of the shell or test like a ring. From this circular 

 canal there radiate, like the arms of a star-fish, certain 

 other canals which pass in front of the rows of per- 

 forated plates, and meet together at the top. Each 



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