ir. ECniNOIDEA. 



345 



but elsewhere it is a double spiral. It ascends from the Biouth, 

 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. Excejit in 

 the Sjjatangoids the mouth is surrounded by 

 fire sharp-pointed calcareous plates, the teeth, fig. 332. — Aristotle's 

 which in the Echinoids are supported by a 'Taurlfvsnvfa^i".^'^(A!t- 

 complicated system of leyers, fulcra, and mus- d'?rto?'^™'^'aiveoii'; ""X 

 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 off 

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

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



Fig. 333. — Oral {A^ and aboral (S) surfaces of the sand dollar. Echinaracltnius pai-ma. 

 a, anus ; y, genital pores ; i, ambulacral areas ; vt, madreijorite ; o, mouth. 



the inner side of the test accompanied by nerves which radiate 

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

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

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



Order I. Paleeechinoidea. 



Paleozoic forms with five ambulacral areas, the interambulaorul areas 

 containiug more tlian two rows of plates. Meloidtts. 



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* StrongyTocentrotus* Arhacia* Calopleurus* (flg. 328). 



