Generalized hemichordate. Colors used here to emphasize the different body sections.
Hemichordata is a small phylum of worm-shaped, marine invertebrates. Hemichordates are bilaterally symmetrical and their bodies are divided into three sections: Protosome or proboscis, collar, and trunk. The musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that covers the inside.
Hemichordata includes about 100 living species. These are widely distributed—being found both in shallow coastal waters and in the deeper sea—and are benthic (live on the sea floor), at least in their adult form. Hemichordates range from only a few millimeters long to the Balanoglossus gigas, which reaches 1.5 meters (4.7 feet).
Hemichordates are sometimes said to be a link between classical invertebrates and vertebrates, particularly sharing similarities with echinoderms and chordates. As diverse as are living organisms, unity is revealed through their shared common lineage.
Hemichordates are considered a sister group of the chordates and echinoderms. They were once considered part of Chordata, but lack a true notochord. A hollow nerve cord, or notochord, is found in all chordates, even tunicates, and some hemichordates also seem to have a primitive form of notochord (at least in early life). In the embryonic stage, this tubular nerve cord looks like the hollow nerve cord of chordates.
Both Hemichordata and Chordata have gill slits, and primitive fossil echinoderms also show signs of gill slits.
As with the Chordata and Echinodermata phyla, Hemichordata are deuterostomes. As deuterostomes, they have true coeloms (body cavities), with the coelom forming through enterocoely (the mesoderm forms as evaginations of the developed gut that pinch off, forming the coelom); the first opening becomes the anus rather than the mouth as in protostomes; and the early divisions of the zygote occurs parallel or perpendicular to the polar axis (radial cleavage).
Hemichordata is divided into two main classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is based on a single species known only from larvae. One of the suggestions is that the pterobranchs are more basal deuterostomes, while the enteropneusts are an early offshoot of the lineage that lead to Chordata.
The Tree of Life Web Project, involving a collaborative effort of biologists, breaks the classes down into the following families (Maddison 1995):
The acorn worms or Enteropneusta include about 70 known species. As with all hemichordates, acorn worms are benthic and may be deposit feeders (substrate feeders) or suspension feeders (filter feeders).
The acorn worm's body is cylindrical and made up of three main parts: the acorn-shaped proboscis, a short fleshy collar that lies behind it, and the long trunk, which is the rest of the body. The creature's mouth is located at the collar behind the proboscis. One theory is that this 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.
The skin is covered with cilia 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 by cilia movements and body contractions.
Acorn worms breathe by drawing in oxygenated water through their mouth. The water then flows out the animal's gills, which are on its trunk. Thus, the acorn worm breathes about the same way as fish.
Acorn worms are considered more highly specialized and advanced than other similarly shaped worm-like creatures. They 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. Hence, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates.
Some also have a post-anal tail that sometimes shows weak signs of segmentation. An interesting trait is that its three-section body plan is not present in the vertebrates, except from the anatomy of the frontal neural tube, later developed into a brain that is divided into three main parts. This means some of the original anatomy of the early chordate ancestors may still be present even if it is not always visible.
Acorn worms are rarely seen by humans because of their lifestyle. They typically live in 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 often 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). They rarely leave their burrows, which may have several openings. 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. Organic material adheres to mucus on the proboscis and is moved by cilia to the mouth. The mouth can be covered by the collar to avoid eating inorganic or other undesirable items. Some acorn worms live in other environments, such as in vegetation (seaweed or plant roots) or sand in a shell, and specimens in deep water have been observed moving freely across the ocean floor.
Acorn worms have separate genders that release eggs and sperm into the water for external fertilization. In some, eggs develop into free-swimming larvae that look very similar to echinoderm larvae. After a number of weeks, the larvae change into tiny acorn worms and settle on the surface and take on the burrowing lifestyle. Others do not have a larval stage, but develop directly into small juveniles.
Pterobranchia is a class of Hemichordata that live in secreted tubes on the ocean floor, and feed by filtering plankton out of the water with the help of cilia attached to tentacles. Unlike the enterpneusts, the pterobranchs possess only one, or even no, pharylgeal slits and each animal has only a single gonad, while enterpneuts have numerous gonads. The collar has between one and nine pairs of tentacles, each of which has a double row of smaller ciliated tentacles.
There are about 30 known living species in the group. These are small, and range from one millimeter to 12 millimeters.
Pterobranchia was established by Ray Lankester in 1877. It contained, at that time, the single genus Rhabdopleura. Rhabdopleura was at first regarded as an aberrant Polyzoon, but with the publication of the Challenger Report (Cephalodiscus) in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta.
Recent advances in electron microscopy have suggested that pterobranchs belong to the same clade as the extinct graptolites.
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