Fig. 2

The embryonic development of Ph. harmeri. Nomarski images of living embryos of Ph. harmeri at representative stages of development: cleavage (a–e), blastula (f–g), gastrula (h–i) and larva (j–k). The egg undergoes its first radial holoblastic cleavage at 2 hpf (a) and forms a hatching blastula around 6–10 hpf (f). Gastrulation starts at 20 hpf (h) at the vegetal pole of the embryo and results in the flattening of the vegetal surface. At late gastrula stage (30 hpf) (i), the apical organ shifts anteriorly, the archenteron elongates posteriorly and the anterior–posterior axis becomes oblique. At early larva stage (40 hpf) (j), the embryo begins to elongate along the anterior–posterior axis and the blastopore becomes the mouth of the future larva. A thick tissue is formed at the dorsal ectoderm and around the mouth that will form the future pre-oral lobe. The bilateral symmetry is evident. The pre-tentacle actinotroch larva is formed around 60 hpf (k), with a prominent pre-oral lobe, a fully compartmentalized, functional gut and evident tentacle bulbs. h–k depict embryos in lateral view and h′–k′ show embryos in vegetal view. Insets show different focal planes of the embryos. In all panels, anterior is to the left