Twin juvenile rudiment in purple urchin larva

On March 29 2010, the first day of our Embryology class, we started our own cultures of sea urchin embryos. I have had the opportunity to observe the different stages of development of the purple urchin, Strongylocentrotus purpuratus. Last week while looking at this echinopluteus through a compound microscope I noticed something that I have not seen before. There were two juvenile rudiments growing inside the larva. Normally, a single rudiment develops on the left side of the larva. In this specimen however, there is one rudiment on the left side of the larval stomach (greenish oval shape roughly in the middle of the larval body) and another rudiment (with protruding tube feet) on the right side. This is a ventral view and the larval anterior is up.

A week later I checked on the twins to see how they were developing. The juvenile urchins have grown very large and the larval body mostly degenerated. Three of the remaining larval arms are visible on the upper right in the bottom picture. Because this specimen was so thick and non-transparent it was difficult to get a clear picture in the transmitted light. This picture  is taken using cross-polarized light to make the juvenile skeletal spicules glow. You can see the spines from the two different juvenile rudiments - some to the left and others to the right of the plane of bilateral symmetry of the larva (which cuts across from the upper right to the bottom left of the picture). The larger of the two rudiments in the first picture was more developed and is now mobile on its tube feet. The other rudiment has spines, but I could not see any tube feet stick out.

The case of twins in this situation intrigues me very much. I am a fraternal twin myself, which means that my twin sister came from a different fertilized egg. These urchin twins came from a single fertilized egg, but they are conjoined (share a single gut). They each have an oral side, but have no aboral side. Likely they will not survive much longer after metamorphosis (S. A. Maslakova, pers. communication).