IV. ECHINOIDEA. 



345 



but elsewhere it is a double spiral. It ascends from the mouth* 

 turning once, and then, bending on itself, coils 

 in the reverse direction to the anus (fig. 331). 

 Usually the first portion of the canal is accom- 

 panied 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 calcareous plates, the teeth, FTG. 332. Aristotle's 

 which in the Echinoids are supported by a l ^rofusiimd^ l (^t- 

 complicated system of levers, fulcra, and mus- d?i? h a ^iveoU; r i 

 cles, the 'lantern of Aristotle' (fig. 332). 



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

 lantern, the stone canal and ovoid gland (' heart') extending 

 upwards from them (fig. 330). The blood-vascular ring gives oft 

 two blood-vessels which run along the alimentary canal, while from 

 the ring canal arise five ambulacral or radial canals which run on 



A 



iurl 

 a, anus ; g, genital pores ; i, ambulacral areas ; m, madreporite ; o, mout 



FIG. 333 Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius par 



ith. 



the inner side of the test accompanied by nerves which radiate 

 from a nerve ring. The gonads are five (rarely four or two) 

 unpaired organs in the aboral half of the test, opening through the 

 genital plates, that is, interradially as in the starfish. 



Order I. Palaeechinoidea. 



Paleozoic forms with five ambulacral areas, the interambulacral areas 

 containing more than two rows of plates. Melonites. 



Order II. Cidaridea (Regulares). 



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

 anus polar. Here belong the common urchins, represented on our coasts, 

 by Toxopneustes* Strongylocentrotus,* Arbacia* C&lopleurus* (fig. 328). 



