73 



there would be a pentagonal opening in the centre. But 

 if we were to grind away the angles of the tips, keeping 

 the edges still sharp, we should have just such an appa- 

 ratus as is shown in Fig. 4. 



The five teeth and their corresponding sheaths are so 

 firmly bound together by the muscles and ligaments, that 

 the entire apparatus can be easily removed without sepa- 

 rating its constituent parts. The whole of this framework 

 is bound to the interior of the Urchin by means of liga- 

 ments which are attached to five strong staples made of 

 the same calcareous matter as that of which the shell is 

 composed. These staples are set round the orifice 

 through which the teeth protrude, and are shown in 

 Fig. 5. 



After the death of the animal decay rapidly sets in, 

 so that if the shell be broken the entire dental framework 

 falls out. This framework is often found on the sea-shore, 

 and is popularly known by the name of "Aristotle's 

 Lantern." 



