Plate VII. 



o: odd spicule characterizing the Qiature Pluteus. 



p: water-pore, 



w: hydrocoel. 



x: communication of the ectodermic invaginated sac with the exterior. 

 Fig. 92. Sketch of the left side of a Pluteus about twelve days old to show the reciprocal position 



of the hydrocoel and the invaginated sac. Dorsal view. 

 » 93. The hydrocoel and invagiaated sac in a Pluteus slightly more advanced. 

 » 94. Sketch of the left side of a Pluteus somewhat older than in fig. 92. Dorsal view. 

 D 95. The same in a more advanced Pluteus. 

 » 96. The same in a still older Pluteus. 

 i> 97. Four developmental stages of a calcareous plate. 



j> 98. A more developed plate in a young sea-urchin. Upper view. Diameter: 0,07 mm. 

 » 99 a — k. The developmental stages of the spines. 



