Morphology and behaviour of Maritigrella crozieri larvae. Images (A) through (L) show larvae 1 day after hatching. (A) through (E), (H) and (L) are extended-focus images produced using Helicon Focus software (HeliconSoft, Kharkov, Ukraine). (F), (G) and (I) through (K) are single-focus images. (A) Ventral view. (B) Anterior view. (C) Posterior view showing the mouth opening (asterisk). (D) Right-side view. (E) Dorsal view. (F) Long cilia (arrow), and large cells (asterisk), are found in the apical tuft region. (G) View of the focal plane illustrated by a dashed line in (J) showing the mouth opening (asterisk). (H) A rosette of cells surround the long cilia of the apical organ (arrow). (I) Short cilia cover the epidermis (asterisk), while long cilia cover the ciliary band cells (arrow). (J) Leftside view showing the mouth opening (arrow). Top, left and bottom white squares are enlarged in (F), (J) and (K), respectively. (K) Long cilia of the posterior tuft. (L) Anterior view. Epithelium with rhabdites surround the apical organ. White square is enlarged in (H). Cerebral and epidermal eyes are marked with arrows and an asterisk, respectively. (M) Trajectories of larvae swimming in a right-handed helical motion towards a light source. Trajectories were deduced from a movie of 2-day-old larvae swimming in an embryo dish (Supplementary movies 3 and 4) and using the MTrackJ plugin  of Fiji software (http://www.fiji.sc). (N) Larval shapes vary from roundish (asterisk) to elongated (arrows) due to muscular contractions. Yolk granules are marked with an asterisk. (O) Left side view of a 5-week-old larva. Note the decreased size, presence of the posterior tuft (arrowhead) and the resorption of lobes. Scale bars are 50 μm in (A) through (E), (G), (J), (L) and (O) and 10 μm in (F), (H), (I) and (K).