G. Cut with a pair of scissors round the " equator line " of the echinus, and 

 separate the two halves without tearing across the intestine, and lay the upper 

 poition beside the lower with aboral surface downwards. Make a sketch 

 showing — 



(a) The dentary apparatus, or Aristotle's Lantern. 



(6) The alimentary canal passing from the lantern arid coiling round the test, 

 to terminate in the centre of the aboral region. 



(c) The generative organs hanging in lobes from the test around the anal aper- 



ture, and opening by five ducts to the exterior. 



(d) The stone canal which passes from the madreporic tubercle in a direct central 



line to end in the water ring surrounding the cesophagus. 



(e) The polian vesicles, 



(/) The five radial water vessels wliich pass down outside the alveoli to reach 



the ambulacral area. 

 (g) The pedicels springing from tlie radial vessel, and the ampullae formed on the 



tube which projects into the test from the bases of the pedicels. 

 (h) The membraneous sinus which runs alongside the upper part of stone canal. 



H- Sketch the dentary apparatus in position from the side and from above, 

 showing — 



(a) The five pairs of long triangular alveoli connected by interalveolar muscles. 

 \b) The teeth l^^ing between the alveoli. . 



(c) The radii extending across the base of the lantern. 



(d) The rotulse articulating at their outer ends with the alveoli, and at their inner 



with the radii. 



(e) The transverse muscles connecting the radii. 



(/) The protractor muscles e.xtending from the top of each alveolus to the inter- 



anibulacral areas. 

 (g) The retractor muscles extending from the oral ends of the alveoli to the auricles. 



I. Cut away carefully with scissors the tips of the alveoli on one side of the 

 lantern, to expose the pentagonal ridge, which forms the nerve ring, on the inner 

 surface of the oral membrane. Trace the radial nerves from the nerve pentagon 

 along the ambulacral areas to their termination in the eye-spots on the ocular plates. 



J. Eemove a portion of the nerve cord and examine with the microscope (low 

 power), noting the lateral branches, and also the radial water vessel and its lateral 

 prolongations. 



