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 vessles that contain the blood- rather is just washes over the organs. They are coelomates and have a a complete digestive tract.
In the pictures below, you can see in the first image that the acorn worm is expelling the sediment after it has removed the nutrients from it. If you then look at the last photo, what you see is exactly what it looks like... worm poo. Only this worm excrement is almost entirely just the sediment that was taken into the mouth. The nutrients were digested from it as the cilia which line its digestive cavity moved the sediment along.
In the pictures below, you can see in the first image that the acorn worm is expelling the sediment after it has removed the nutrients from it. If you then look at the last photo, what you see is exactly what it looks like... worm poo. Only this worm excrement is almost entirely just the sediment that was taken into the mouth. The nutrients were digested from it as the cilia which line its digestive cavity moved the sediment along.