Acorn worm

as well as glands that secrete mucus. Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms move only sluggishly, using ciliary action and peristalsis of the proboscis. Research indicates that the rate of feeding of acorn worms that are detritus feeders is dependent on food availability and flow rate. A groove lined with cilia lies just in front of the mouth and directs suspended food into the mouth and may allow the animal to taste. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft and to the exterior through a dorso-lateral pore (see diagram below). Cilia push water through the slits, maintaining a constant flow. The tissues surrounding the slits are well supplied with blood sinuses. There are, however, numerous nerve endings throughout the skin. Similarities to chordates Acorn worms have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates. Some also have a postanal tail which may be homologous to the post-anal tail of vertebrates. An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain which is divided into three main parts. This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible. One theory is that the three-part body originates from an early common ancestor of all the deuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes. Studies have shown that the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system, they are controlling the development of the different body regions. ==Phylogeny==

The internal relationships within the Enteropneusta are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. }} ==Lifestyle==

''. Acorn worms are rarely seen by humans because of their lifestyle. They live in U-shaped burrows on the sea-bed, from the shoreline down to a depth of 10,000 ft. (3,050 m). The worms lie there with the proboscis sticking out of one opening in the burrow. Acorn worms are generally slow burrowers. To obtain food, many acorn worms swallow sand or mud that contains organic matter and microorganisms in the manner of earthworms (this is known as deposit feeding). At low tide, they stick out their rear ends at the surface and excrete coils of processed sediments (casts). Another method that some acorn worms use to obtain food is to collect suspended particles of organic matter and microbes from the water. This is known as suspension feeding. ==Reproduction==

Acorn worms are dioecious, having separate biological sexes, although at least some species are also capable of asexual reproduction in the form of fragmentation. They have paired gonads, which lie close to the pharynx and release the gametes through a small pore near to the gill slits. The female lays a large number of eggs embedded in a gelatinous mass of mucus, which are then externally fertilized by the male before water currents break up the mass and disperse the individual eggs. In most species, the eggs hatch into planktonic larvae with elongated bodies covered in cilia. In some species, these develop directly into adults, but in others, there is a free-swimming intermediate stage referred to as a tornaria larva. These are very similar in appearance to the bipinnaria larvae of starfishes, with convoluted bands of cilia running around the body. Since the embryonic development of the blastula within the egg is also very similar to that of echinoderms, this suggests a close phylogenetic link between the two groups. After a number of days or weeks, a groove begins to form around the larval midsection, with the anterior portion eventually destined to become the proboscis, while the remainder forms the collar and trunk. The larvae eventually settle down and change into tiny adults to take on the burrowing lifestyle. A few species, such as Saccoglossus kowalevskii, lack even the planktonic larval stage, hatching directly as miniature adults. ==References==