On April 13th 2011 I found a pilidium larva in a plankton tow collected off the F dock in the marina in Charleston, OR. A pilidium is a planktonic larva of some nemertean worms, predominately from the order Heteronemertea. Pilidium larvae form eight juvenile rudiments which will eventually make up the adult worm (Maslakova 2010). These rudiments, called imaginal discs, fuse around the larval gut and form the juvenile worm. In the pilidium larva there are three pairs of imaginal discs, plus two unpaired rudiments (the dorsal disc, and the proboscis rudiment).
These two pictures show the same larva in two different focal planes. This particular larva has developed two pairs of imaginal discs and the proboscis rudiment. Larval anterior, marked by the apical plate, is up. In the first image one of the cephalic discs and one of the trunk discs is in focus. The cephalic discs (to the right on this picture) give rise to the head of the juvenile worm, including the cerebral ganglia. The trunk discs are to the left on this image, directly underneath the stomach, which the large round shape inside the pilidium larva. The trunk discs give rise to the vast majority of the trunk ectoderm of the juvenile. On the second image the proboscis rudiment is in focus - it is the small round blob underlying the pilidial epidermis almost directly above the cephalic discs (at about 2 o'clock in this picture). It will fuse with the cephalic discs and form the proboscis in the adult worm. Paired cerebral organ discs and an unpaired dorsal rudiment also form in the pilidium larva, but these are not yet developed in the photographed larva. The cerebral organ discs give rise to the cerebral organs (chemosensory structures), while the dorsal rudiment contributes to the formation of the adult ectoderm.
S. A. Maslakova. 2010. Development to metamorphosis of the nemertean pilidium larva. Frontiers in Zoology 7:30