The planuliform larva is uniformly ciliated and has a prominent bundle of cilia called the apical tuft at the anterior end (12 o’clock on the second picture). Such larvae are found in nemerteans (phylum Nemertea, a.k.a. ribbon worms) from the orders Hoplonemertea and Palaeonemertea.
After a few more days these larvae developed features that allowed me to identify them as belonging to the order Hoplonemertea (e.g. several pairs of subepidermal eyes visible on the third picture). This picture shows an 11-day old individual. At this point they mostly crawled on the bottom of the dish, and had developed many of the adult structures, such as the brain and proboscis, so they can be considered juveniles rather than larvae. Hoplonemertean metamorphosis (the transition from planktonic larva to benthic juvenile) is inconspicuous. The transition from swimming to crawling is accompanied by changes in the epidermis. Apparently many hoplonemerteans replace the larval epidermis composed of large, ciliated, cleavage-arrested cells with intercalating smaller cells of the definitive epidermis (Maslakova and von Döhren, 2009).
To witness this epidermal transition, I fixed some of my specimens in paraformaldehyde (with a touch of gluteraldehyde) and stained them with fluorescent phalloidin to visualize the outlines of the epidermal cells using a confocal microscope.
The first confocal image shows a 2-day old hoplonemertean planuliform larva – the same age as the live larva pictured above. You will notice that large cells dominate the epidermis, but small cells are visible in between. These small cells are the intercalating cells of the juvenile epidermis. The bottom image is a 4-day old larva, and you can see that the large cells of the larval epidermis are farther apart from each other. The small cells of the juvenile epidermis occupy more space in between. Eventually, cells of the larval epidermis will be either resorbed or sloughed off, leaving the cells of the definitive epidermis to cover the entire surface.
Maslakova SA, von Döhren J. (2009) Larval development with transitory epidermis in Paranemertes peregrina and other hoplonemerteans. Biol Bull 216: 273-292