54 



Invertebrata. 



cellariae, somewhat like those of starfishes ; the oppo- 

 site or aboral pole is surrounded by five plates, each 

 placed at the end of one of the imperforate meridians, 

 and each pierced by the end of the duct from one of 

 the five large egg-secreting glands which lie within 

 the shell between the ambulacra. Between these 

 ovarian plates and at the end of the ambulacra are 

 five smaller plates, each pierced by the end of the 

 radial nerve threads, and bearing a little eye-speck ; 

 these are called ' ocular plates.' One of the former set 

 of plates is always unsymmetrical, swollen, and finely 

 pierced with holes, being in fact the combination of an 

 ovarian plate with the madreporiform tubercle at the 

 end of the sand canal as in starfishes. 



Sea Urchins have ambulacral vessels just like those 

 of starfishes, with a tubular ring around the mouth and 

 FlG . 32 . five branches, one 



along the inner sur- 

 face of each set of 

 perforated plates, 

 through the holes in 

 which the tube feet 

 project. 



Most Sea Urchins 

 have the mouth 

 armed with a com- 

 plex system of teeth, 

 five of which are 

 placed around the 

 orifice with their 

 points directed to- 

 wards it, each being situated in the centre of a wedge- 



Tooth apparatus of the Sea Urchin, showing 

 the arrangement of the muscles. 



