xiv] 



ECHINOIDEA 



347 



smooth rounded bosses on the plates (9, Fig. 160, 8, Fig. 164 B). 

 They are tied to these bosses by a sheath of muscle-fibres, so that 

 by the special contraction of any side of the sheath they can be 



. 20 



19 



13 



11 



FIG. 162. Aristotle's Lantern of E. esculentus x 2. Partly from Chadwick. 



Upper end of tooth enveloped in lantern membrane. 2. Compass. 



3. Elevator muscle of compasses. 4. Depressor muscles of compass. 



5. Jaw. 6. Retractor muscles of the jaws. 7. Protractor of jaws. 

 8. Auricula. 9. Ampullae of tube-feet. 10. Inter-ambulacral plate. 

 11. Tooth. 12. Circular water-vascular vessel. 13. Epiphysis. 



14. Polian vesicles. 15. Oesophagus. 16. Ventral "blood "-vessel. 

 17. Genital stolon. 18. Stone-canal. 19. Rectum. 20. Madre- 

 poric plate. 



