Figure 1. Hemichordate adult (left panel) and juvenile (right panel). The acorn worm, Saccoglossus kowalevskii, is shown. On the left, bar 1 cm; and on the right bar, 1 mm.

Figure 2. Phylogeny of the hemichordates, based on DNA sequence comparisons. Note that hemichordates, echinoderms and chordates share a common ancestor, the deuterostome ancestor.

Figure 3. Developmental stages of S. kowalevskii. Drawings are modified from the 1884 publication of William Bateson, and they have been arranged in order and labelled. The temperature of development is approximately 22°C.

Figure 4. Fate map of S. kowalevskii. A schematic drawing of the 8-celled embryo, which the Colwins (1953) marked with a Nile blue spot at various places. Embryos were then allowed to develop to the juvenile to see what tissues and organs were blue. The initial and final positions of the marks are related by the fate map.

Figure 5. The similar domain map of hemichordates (above) and chordates (below). Each domain is the specific region of the embryo where a particular gene is expressed. These genes encode proteins of importance in the animal's development. Lines are drawn vertically to set off anterior, middle and posterior sections of the map, to show that the genes expressed in the chordate forebrain are expressed in the hemichordate prosome, that genes expressed in the chordate midbrain are expressed in the hemichordate mesosome and anterior metasome back to the first gill slit, and that genes expressed in the chordate hindbrain and spinal chord are expressed in the hemichordate metasome posterior to the first gill slit.

Figure 6. The tornaria larva. A schematic diagram of the larva of Balanoglossus claverigus, side view. The thick black lines indicate rows of cilia, one around the mouth for feeding and one near the anus for movement. Depending on the species, these rows may be more elaborate with protruding loops.