Hemichordata
| Hemichordata | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Acorn worm Generalized hemichordate. Colors used here to emphasize the different body sections. | ||||||||||
| Scientific classification | ||||||||||
| ||||||||||
| Classes | ||||||||||
Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the chordates. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. They seem to have a primitive form of notochord, but this is most likely the result of convergent evolution. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes. The musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that covers its inside. They are tradtionally considered as a sister group to the echinoderms.
Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is monophyletic is currently under debate. One of the suggestions are that the pterobranchs are more basal deuterostomes, while the enteropneusts are an early offshoot of the lineage who are leading to Chordata.
ReferencesISBN links support NWE through referral fees
Enteropneusta (Acorn worms)
The Acorn worms or Enteropneusta are a hemichordate class of invertebrates. Acorn worms are classified in the phylum Hemichordata, closely related to the chordates. There are about 70 species of acorn worm in the world.
All species are infaunal benthos that either may be deposit feeders or suspension feeders. Some of these worms may grow to be very long; one particular species may reach a length of 2.5 meters (almost eight feet), although most acorn worms are much, much smaller.
One genus, Balanoglossus, is also known as the tongue worm.
Anatomy
The Acorn worm's body is cylindrical. The body is 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.
Similarities to Chordates
Acorn worms are considered more highly specialised 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 postanal tail which sometimes show weak signs of segmentation. An interesting trait is that its three-section body plan is no longer present in the vertebrates, except from 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.
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 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 (this suggests vertebrates and echinoderms are linked phylogenically). The larvae eventually settle down and change into tiny acorn worms on the surface and take on the burrowing lifestyle. Others don't have a larval stage, but develops directly into small juveniles.
Pterobranchia
Pterobranchia is a clade of small, worm-shaped animals. They belong to the hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group.
Pterobranchia were 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.
References
- This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
- At Billie Swalla's site
- Cameron, C. B. Classification of the extant hemichordata
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
