Lesson 9: Hemichordata: Acorn Worms
Hemichordates are considered to be the possible "missing link" between chordates and other animals. Hemichordates share the same basic developmental characteristics as echinoderms and chordates. They have a dorsal, hollow nerve chord and openings along the anterior part of the gut (gill slits that you will learn more about later). There are 85 known species and they are deposit feeders that live free or in u-shaped burrows. Some are found near hydrothermal vents. Most are fairly small, ranging from 8-45 cm but some can reach 2.5 m (8 ft!).
Similarly to the sea cucumber, acorn worms ingest sediment using a thick mucus covered proboscis to collect organic material. They have a mostly open circulatory system, which means that there are no vessels that contain the blood- rather is just washes over the organs. They are coelomates and have a complete digestive tract.
Photo Attribution
Description: Acorn Worm
Source: Wikimedia.org