Fig. 1

Development: light micrographs of cleavage and gastrulation in Schizocardium karankawa. A Zygote with polar bodies. B 2-cell stage. C 4-cell stage. D 8-cell stage. E 32 cell stage. F Blastula. G Mid-gastrula. H Hatching gastrula. I–K Early larval development in Schizocardium karankawa. I Lateral view of 23 hpf stage embryo at the time of mouth, stomach, and apical tuft formation. J Lateral view of developing tornaria larva at the time of protocoel formation. K Dorsal view of tornaria larva with mesocoel and metacoel, 30-day post-fertilization. L Late-stage larva with gills, 50-day post-fertilization. M Newly settled juvenile worm retained the larval pigment cells and the apical organ, feeding bands and telotroch. The larval apical organ develops into an outward projecting exploratory organ. N Juvenile worm showing the formation of the proboscis, collar, and trunk, 68-day post-fertilization. O Juvenile worm with the three regions of the trunk (branchial, hepatic, and intestine), 85-day post-fertilization. P Adult worm with hepatic sacs. a archenteron, af anal field, an anus, ao apical organ, ap apical plate, at apical tuft, bc blastocoel, br branchial region, c collar, gb gill bars, hr hepatic region, hs hepatic sac, i intestine, ldl lower dorsal lobe, m mouth, met metacoel, mes mesocoel, p protocoel, pc pigment cells, pdl primary dorsal lobe, pds primary dorsal saddle, pre preoral loop of the circumoral ciliary band, post postoral loop of the circumoral ciliary band, pp protocoel pore, pr proboscis, tt telotroch. Scale bars: A–H = 150 μm; I, J = 200 μm; K, L = 1150 μm; M = 3300 μm; N = 2100 μm; O = 2500 μm; P = 2 cm