Parts of the Lantern of Aristotle in Echinus miliaris. 485 



DESCRIPTION OF PLATES. 

 Plate 11. 



Fig. 1. — "Echinus rudiment" of a metamorphosing plutens ; macerated after all the 

 extra-larval parts have been removed by sectioning ; the lantern elements are seen 

 from the future aboral surface ; they are all, with the exception of the teeth, in 

 the stage of tri-radiate spicules ; no trace of compasses. 



Fig. 2. — Aboral view of the lantern elements in a young imago, probably just meta- 

 morphosed ; the top part of the shell has been removed and the specimen macerated ; 

 the inner rudiment of a compass visible in one radius ; branching of the rays has 

 commenced in all the spicules. 



Plate 12. 



Fig. 3. — Aboral view of the same in an imago-urchin slightly more advanced ; well- 

 marked rudiments of the compasses are laid down in all the radii ; epiphyses 

 considerably branched ; the teeth are rather diagrammatically represented. 



Plate 13. 



Fig. 4. — Aboral view of the same in imago still more advanced ; the first branches of 

 each rotular rudiment have joined end to end and fresh off-shoots have been put 

 forth ; one ray of the inner rudiment of each compass is in process of resorption 

 and another ray is being lengthened to meet the outer rudiment ; beaks are not 

 formed by the earlier-produced cones ; the root-end of a tooth shows the 

 characteristic bend of the same seen in the adult ; a compass has been omitted 

 in one radius ; it will be noticed the specimen is tetramerous in symmetry. 



Plate 14. 



Fig. 5.— Aboral view of the same in imago more advanced than the one shown in 

 fig. 4 ; the rotulae and the epiphyses have grown into fenestrated plates ; the 

 outline of the original spicule may yet be traced in each rotula ; the two rudiments 

 of each compass are still separate ; the epiphyses are putting forth processes ; the 

 alveoli are not visible ; the compasses in two radii and a rotula in one radius have 

 been omitted. 



Plate 15. 



Fig. 6. — Aboral view of the same in a fairly grown young urchin ; the rotulse are 

 beginning to sink between the two epiphyses below ; the off-shoots of the latter 

 have nearly met across the "foramen magnum"; the two pieces of each compass 

 have met and formed a suture ; a carina in each tooth can now be distinctly seen ; 

 the jaws are not visible ; a rotula and a compass have together been omitted in 

 one radius in order that a pair of epiphyses may be brought into view ; likewise, 

 the teeth have been left out in two radii -to show the bridge formed by the 

 processes of two epiphyses ; the compass alone has been omitted in one radius to 

 bring a rotula into full view. 



2 m 2 



