IV. ECHINOIDEA 



305 



the anus (fig. 304). Usually the first portion of the canal is accompanied 

 by a siphon, an accessory tube opening into the main tube at either end. 

 Except in the Spatangoids the mouth is surrounded by five sharp-pointed 



ed 



\ 



nd d 



A 



st 



FIG. 304. Sea urchin opened around the equator. A, ambulacral area; 7. intrr- 

 ambulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd, 

 siphon; oe, oesophagus; p, p', ring canal and Polian vesicles; st, stone canal. 



calcareous teeth, which in the Regularia are supported by a complicated 

 system of levers, fulcra, and muscles, the 'lantern of Aristotle' (fig. 305). 

 The ring canal and the ring of the blood system lie on the lantern, 

 the stone canal and septal organ ('heart') extending upwards from them 

 (fig. 303). The blood-vascular ring gives off two 

 blood-vessels which run along the alimentary 

 canal, while from the ring canal arise five ambu- 

 lacral or radial canals which run on the inner side 

 of the test, accompanied by nerves which, enclosed 

 in a tube of infolded ectoderm, radiate, from a 

 nerve ring. The gonads are five (rarely four or 

 two) unpaired organs in the aboral half of the test, 



FIG. 305. Aristo- 

 tle's lantern of Stron- 

 Ih'idus 



opening through the genital plates, that is, interra- ^^f ^aSli; I', 

 dially as in the starfish. teeth. 



Order I. Palsechinoidea. 



Paleozoic forms with five ambulacral areas, the interambulacral areas con- 

 taining more than two rows of plates. Melonites. 



Order II. Cidaridea (P^egularia). 



Ambuiacral areas band-like, body more or less spherical, mouth and anus 

 polar. Common urchins; Toxopneustes* Strongylocentrotus* Arbacia* Ccelo- 



pleurus* (fig. 301). 

 20 



