Stage with four pairs of arms.

Ectoderm.

Early eight-arm stage. Well developed vestibule.

Growth of antero-lateral pair of arms (2nd pair).

View of the animal pole.

Larva with six arms.

Growing postero-dorsal arms.

Stage with three pairs of arms.

Early stage with two arms (post-oral).

Stage with four pairs of arms.

Stage with four pairs of arms.

Stage with four pairs of arms.

Stage with three pairs of arms.

Ring of mesenchyme cells beginning to secrete the calcareous skeleton.

Tip of a post-oral arm in a two-arm pluteus larva.

Aboral region.

Vestibule in an eight-arm pluteus larva.

Four-arm stage.