1. The larva of Spatangus purpureus reaches its last stage, which is characterized by its possession of six pairs of arms, in the course of three weeks after fertilization.
2. The paired arms develop in the following order: postoral, antero-lateral, postero-dorsal, pre-oral, postero-lateral, and antero-dorsal. The posterior process appears about the same time as the antero-lateral arms become distinct.
3. These six pairs of arms and the unpaired process are each supported by a calcareous rod. Of these calcareous rods one can distinguish two classes which differ morphologically from each other, viz. the simple and the composite.
4. To the class of simple rods belong the antero-lateral, pre-oral, postero-lateral, and antero-dorsal rods. They are either direct prolongations or branches of the three arms produced from one of the calcification centres. They are originally horizontal (parallel to the surface of the body) in position, and are homologous with the body-, recurrent, and horizontal rods
5. The remaining rods, viz. the post-oral and postero-dorsal rods and the aboral spike (posterior rod) are composite. They are each composed of three parallel rods connected by transverse beams so as to give a latticed appearance. Each of the parallel rods is a branch given out vertically from an arm of the calcification centre.
6. The larval skeleton of Spatangus pur pure us is characterized chiefly by (a) more or less considerable length of the unfenestrated proximal portions in the latticed rods, (b) fusion of the tips of the ventral horizontal rods forming thickened joint, (c) overlapping of the bocty-rods near their posterior ends, and subsequent fusion of this part so as to form an oblique cross, (d) rather simple appearance of the postero-lateral rods, and (e) formation of a calcareous plate on each side of the stomach in the oldest stage.