Echinospira larvae with “double shell”.

Trivia monacha
1 & 2 middle stage larva.
1: aperture of echinospira shell seen edge on; diameter 0.8mm.
2: two velar lobes used to maintain position of larva in water column.
3, 4, 5 & 6 fully developed larva.
3: slight lateral bay, but insufficient to change velum into four lobes.
4: aperture of echinospira shell seen edge on; diameter 1.25mm.
5 & 6 echinospira shell seen from side, diameter 1.25mm.
5: flimsy, transparent, shiny, exterior shell (Fretter & Graham, 1962, interpreted it as periostracum layer separated from calcareous shell) filled with seawater; decreases specific gravity of veliger to neutral buoyancy, and increased surface area slows rate of sinking, so easing effort needed to orientate and maintain position in water column. (McCloskey, 1972).
6: internal calcareous shell containing animal.

Trivia arctica
7 & 8 middle stage larva.
7: aperture of echinospira shell seen edge on; diameter 0.96mm.
8: indentation on velum starting to divide it into lobes.
9, 10, 11 & 12 fully developed larva.
9: aperture of echinospira shell seen edge on; diameter 1.6mm
10: deep indentation on velum, larva now four-lobed.
11 & 12 echinospira shell seen from side, diameter 1.6mm.
11: exterior shell/separated periostracum filled with seawater.
12: internal calcareous shell containing animal.

Though T. monacha has larger adults than T. arctica its larvae are smaller at equivalent stages and its velum is less developed probably because it is more coastal and needs less effort to maintain position in the water column.
More detailed information in Lebour 1931 & 1933 (Links to PDFs in references on main account).

Redrawn and adapted from Lebour 1931 and 1933.