Arm formation in pluteus larvae

I was interested in how the sand dollar pluteus larva develops. Here are a few pictures of successive developmental stages of this species, Dendraster excentricus. The top picture shows an early four-armed pluteus larva which is three to four days old. It has formed the first two pairs of arms called the post-oral and antero-lateral arms. The post-oral arms form first and are the longest. One can also see the calcareous spicules supporting the antero-lateral arms.

The next photo shows a 14-day old larva which is more advanced. It has longer antero-lateral arms, which project outwards from the anterior end and frame the mouth of the pluteus. The formation of extra arm pairs extends the length of the continuous ciliated band which surrounds the larval mouth and is used to capture microscopic food particles. The longer the ciliated band, the more efficient pluteus can feed. The small bumps at the base of the post-oral arms are the newly developing postero-dorsal arms.

Although one can barely see them, one can already distinguish the small calcareous spicules, which support the new pair of arms, using cross-polarized light. The four spicule rods supporting the antero-lateral pair of arms (towards the midline) and the longer post-oral pair of arms are clearly visible. The shorter postero-dorsal spicule is visible on the left side.

The final pair of arms to form are the pre-oral arms, which, true to their name, form just anterior to the mouth of the pluteus larva. These arms are more or less parallel to the anterolateral arms, and can be seen as small bumps between the antero-lateral arms. On this bottom picture you can also see an unpaired rudiment of the juvenile sand dollar (a bean shaped mass to the left of the larval stomach - which is a large darkish oval occupying the majority of space inside the larval body.