Table 1 Description of larval stages of the pilidium of Micrura alaskensis based on certain developmental milestones.

blastosquare

flat square blastula, swims with cilia (Figure 2A-B, D)

16 hours

gastrula

radially symmetrical swimming stage with an apical tuft and a vegetal invagination (Figure 2C)

24 hours

young pilidium

bilateral symmetry apparent in the gut, longer cilia along the vegetal margin, nascent lateral lappets (Figure 2E-H)

40 hours

feeding pilidium

helmet-shaped pilidium with a ciliated band spanning the lappets, capable of capturing unicellular algae (Figure 3A)

66 hours

cephalic discs

a pair of cephalic discs is present (Figures 3B, 6)

7 days

trunk discs

a pair of trunk discs is present (Figure 3C, 7)

9 days

cerebral organ discs

a pair of cerebral organ discs appears near the trunk discs at the same time as the proboscis rudiment (Figures 3D-E, 8)

14 days

head and trunk

the proboscis rudiment fused with the cephalic discs to form the head rudiment; cerebral organ discs fuse with the respective trunk discs to form the trunk rudiment; an unpaired dorsal rudiment appears near the larval stomach, it is initially separate from the trunk discs and later fuses with them (Figure 3F-G, 9E)

24 days

torus

head and trunk rudiments fuse forming a toroid of tissue around the larval gut (Figure 3H, 9F)

28 days

extended proboscis

proboscis rudiment extends beyond the dorsal margin of the juvenile head, but has not yet reached the stomach (Figures 3I, 9G)

NA

complete proboscis

proboscis reaches the stomach and contacts the dorsal margin of the juvenile trunk epidermis (Figures 4A, 9H)

NA

hood

dorsal margin of the juvenile trunk extended over the posterior portion of proboscis, but has not yet reached and fused with the dorsal margin of juvenile head (Figure 4B)

NA

metamorphosis

fully formed juvenile emerges from the larva and devours it (Figure 5, see also additional file 1 - Movie 1)

35 days