Plate VIII. 



Fig. 100. Dorsal view of a Pluteus about forty-five days old, showiug the spines and feet of tbe 

 young sea-urchin protruding from the left side, o: odd calcareous spicule surrounding 

 the water-pore, p, and supporting the anterior ventral arms of the Pluteus. 



» 101. Side view of a very young sea-urchin forty-five days old. Diameter: 0,24 mm. 



B 102. Dorsal view of another sea-urchin of the same age. Length: 0,26 mm. 



B 103 — 104. Dorsal views of two minute sea-urchins of about the same age and siie, showing 

 the remains of the larval spicules in a state of resorption and the change of the odd 

 spicule, 0, into the meshy calcareous plate which gradually increases and finally covers 

 the dorsal surface, p : water-pore. Length, 0,28 mm. Age, about fifty days. Spines, 

 feet etc. are omitted in the figures. 



B 105. The peristomal region seen from the under surface, showing the development of the den- 

 tary apparatus in a young sea-urchin about fifty days old. The buccal membrane is 

 entire and unpierced. a: frontal anterior ambulacrum, p: posterior odd interambula- 

 crum; t: tooth. 



B 106. A young sea-urchin about forty-five days old, seen from the under surface. Tbe buccal 

 membrane is unpierced. a: frontal or anterior ambulacrum; p: posterior odd interam- 

 bulacrum ; r: radial plates; i: interradial plates; s: spherids in their receptacles; 

 t: teeth. 



