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THE METAMORPHOSIS OF ECHINODERMS. 135 
Figs. 32a and 4.—Two spines with four points from an old pluteus of 
Echinus microtuberculatus; a is the more typical form. xX 260. 
Fie. 33.—Spine with six points from the same larva. x 260. 
Fie. 34.—Very young Echinus microtuberculatus seen from the side. 
x 180. 
Fic. 35.—Diagrammatic view of the same, from a slightly different aspect, 
to show the distribution of the two kinds of spines. 
Fie. 36.—Aboral pole of an older specimen of the same, with a diameter of 
about ‘75 mm. xX 75. 
Fic. 37.—Aboral pole of the same, with a diameter of 7 mm. X 28. 
Fic. 38.—The same, diameter 10 mm. In this stage the madreporic area 
has spread all over the first basal plate. x 23. 
Fic. 39.—Part of a section through a specimen of Echinus micro- 
tuberculatus with a diameter of 3°5 mm., showing the relative positions of 
the water-tube, &c. The ovary marks the centre of interradius A. x 90. 
Fic. 40.—Ventral view of an Ophiurid pluteus (compare 5, fig. 2). The 
water-tube (not seen from this side) enters the hydrocel between pouches I 
and V. x 180. 
Fic. 41.—Section of young Ophiurid just before its separation from the 
remnant of the pluteus. x 500. 
Fic. 42.—Ventral view of Ophiurid pluteus showing the anus pushed to the 
right side. x 300. 
Fie. 43.—The same, later stage. x 300. 
Fic. 44.—Diagram of the bilateral ancestor, from the left side. 
Fic. 45.—The same, dorsal view. 
Fic, 46.—Transition to radial stage, dorsal view 
Fic. 47.—The same, from the original left side. Atrium and tentacles 
omitted. 
Fie. 48.—Radial stage, from the former left (now oral) side. Atrium and 
tentacles omitted as before. For the explanation of the line X—Y, see text. 
Fic. 49.—The same, lateral view. Half the atrium, and two of the tentacles, 
are supposed to have been removed. 
Fic. 50.—Diagram of hypothetical ancestor of Pelmatozoa; the stalk is 
greatly exaggerated (the original form being probably sessile) to show how a 
precocious development of this organ would lead to the conditions seen in the 
larva of Antedon. 
