Fig. 1

Confocal microscope sections of Heliocidaris erythrogramma from the gastrula to the rudiment stage larva. Orientation of larvae is with anterior to the top and posterior, the blastopore, to the base. The left coelom is either on the left (a–d), or the view is of the larval left side with the left coelom in frontal view (e–l). a–b Gastrulae with the archenteron and the left coelom. c–d Rudiment formation in the early larva begins with extension of the left and right coeloms posteriorly from the anterior coelom and the formation of the vestibular ectoderm. e–h Confocal sections through an advanced larva. Development of the anterior portion of the left coelom to form the hydrocoele lobes in the advanced larva. i–l Confocal sections through the same larva, the expansion of the hydrocoele lobes and the overlying vestibular ectoderm form the primary podia that are visible externally. The stone canal and vestibule are also evident. Ar, archenteron; Bp, blastopore; Lc, left coelom; Ac, anterior coelom; Rc, right coelom; Ve, vestibular ectoderm; Lpc, left posterior coelom; Hl, hydrocoele lobes; Pp, primary podia; V, vestibule; H, hydrocoele; Sc, stone canal; Hp, hydropore. Scale bar: 200 μm. See Morris [30] for a detailed assessment of coelomogenesis in H. erythrogramma though analysis of confocal sections