Fig. 3

The embryonic development of Lineus ruber. A–H Photographs of fixed dissected embryos of L. ruber at representative stages of development. A A stereotypical quartet spiral cleavage results in the formation of a yolky coeloblastula. B′, B″ Gastrulation occurs at one pole of the embryo and results in the formation of a central round blastopore and an internal archenteron. C′, C″ The late gastrula shows the first signs of bilateral symmetry, with the future anterior end pointier than the posterior side, which is more round (more evident in C″). D′, D″ The Schmidt’s larva forms after 12 days of development. It exhibits an anterior pair and a posterior pair of ectodermal imaginal discs (arrowheads), and the blastopore has become the mouth of the larva. A thick ectodermal tissue lines the mouth opening. E′–E′′′ The Schmidt’s larva of L. ruber can feed on other siblings that occupy the same egg capsule. The ingestion of this yolk material increases the size of the larva and fills up the blind gut. At this stage, the imaginal discs are still visible (arrowheads) and a conspicuous epidermis covers the larva (arrows). F′, F″ About 18 days after oviposition, the Schmidt’s larva metamorphoses into the juvenile. The imaginal discs (arrowheads) expand and the larval epidermis (arrows) detaches from the body. G′, G″ After metamorphosis, the juvenile shows a worm-like shape, elongated along the anteroposterior axis but not along the dorsoventral axis, mostly because of the high amounts of yolk present in the blind gut. The head region is more developed and the rudiment of the proboscis is now evident. H 8 weeks after oviposition, the juveniles are morphologically similar to the adults, and have absorbed part of the ingested yolk. I Schematic summary of the embryonic development of L. ruber at 14 °C. The light blue area depicts the ingested yolk. Drawings are not to scale. A, B″, C″, D″, E″, E′′′, F″, G″, H show lateral views. B′, C′, D′, E′, F′, G′ show blastoporal/ventral views. In C′–H anterior is to the left. bc blastocoel, bg blind gut, bp blastopore, id imaginal discs, iy ingested yolk, le larval epidermis, mo mouth, pb proboscis, pr pharyngeal rudiment. Scale bars (A–D″) 50 μm; (E′, E′′′, F′) 100 μm; (G′, H) 500 μm