OCULAR SYSTEM. 



Intercalated between the genital plates, at the abactinal extremity of 

 the ambulacra! system, are five plates smaller than the genital plates, which 

 have received the name of ocular plates. They are more or less polygonal, 

 and from their relative positions between the genital and anal plates im- 

 portant generic characters have been derived. These plates are perforate, 

 allowing the passage of an odd tentacle ; this is the homologue of the odd 

 ocular tentacle of starfishes, and although no eye-specks have as yet been 

 discovered in these odd tentacles, they are, from their position at the end 

 of the ambulacra! tube, undoubtedly the homologue of the ocular tentacles 

 of the starfish. For a detailed account of the odd tentacle and its probable 

 homology, as well as the interpretation of the ocular plates, 1 would refer to 

 the description of the young Echini. In the Desmosticha and the Clype- 

 astroids the ocular plates are more prominent than in the Petalosticha. 



