One week after fertilization, I noticed a pair of buds between the post-oral (the longest pair) and the antero-lateral arms, and quickly realized that they were the postero-dorsal arm buds (top picture). I was impressed to see that the calcareous spicules which provide support for the arms were already present in the arm buds. I took a picture two days later using two polarizing filters to highlight the spicules (middle picture).
By two weeks after fertilization, the larvae in my culture had become 6- armed plutei (bottom picture). As I observed and drew this stage, I couldn’t quite figure out how the spicules in the postero-dorsal arm rods connected with the rest of the larval skeleton. I assumed that the spicules in the arms simply branched off the existing tri-radiate spicule, which projects branches into post-oral and antero-lateral arms, as well as the larval body. I was surprised to find out that these spicules are not connected to the rest of the skeleton.