ARBACIA PUNCTULATA 201 



if necessary, and place both halves side by side in the dissecting 

 tray with water. The oral half will contain the Aristotle's 

 lantern, oesophagus, stomach and siphon, part of the axial com- 

 plex (hydrophone canal with the axial organ) and five ambulacral 

 radial canals with ampullae, as well as the ring canal. The 

 aboral half will contain the intestine with the rectum, five 

 gonads and five radial canals with ampullae. Make a full page 

 drawing showing both halves side by side with all organs in situ. 



14. Cut the oesophagus and remove the stomach from the 

 oral half. This will expose the Aristotle's lantern inclosed in 

 the oral sinus. Carefully remove with a needle and fine forceps 

 the wall of the sinus, exposing all parts of the lantern. Make 

 a drawing twice natural size showing the base of the lantern 

 as viewed from above. The drawing must show the transverse 

 section of the oesophagus surrounded by the diaphragm, the 

 compass ossicles with their tendons, the curved ends of the 

 teeth and the auricles with the retractors. 



15. Make a drawing showing the side view of the lantern 

 on a large scale. The draXving must show the curved ends of 

 the teeth, the head of the compass ossicles, the compass liga- 

 ments, the rotulae, the epiphyses with the protractors, the alveoli 

 with the interpyramidal muscles, the retractors and the auricles. 



16. Take out the lantern and separate the alveoli. Make a 

 drawing of an isolated alveolus with the tooth. 



17. Separate the two parts of an alveolus, taking care not to 

 break the tooth. Make a drawing of the tooth showing the 

 cutting edge, the central portion and the elastic end. 



