hemichordate
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What are hemichordates?
Hemichordates are marine invertebrates closely related to chordates and echinoderms. The phylum name, Hemichordata, means "half-chordate," because a structure in the mouth cavity resembled the notochord of primitive vertebrates; however, this has been rejected, and molecular studies indicate a closer relationship to echinoderms.
Hemichordates have three classes: Enteropneusta (acorn worms), Pterobranchia, and Planctosphaeroidea. Adult hemichordates have a body divided into three parts: proboscis, collar, and trunk. They lack a central nervous system. All hemichordates are suspension feeders, capturing minute organisms and algae. They have evolutionary links with chordates and echinoderms, but precise relationships are still being studied.
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Where do hemichordates live?
Hemichordates, a phylum of marine invertebrates, consist of acorn worms and pterobranchs. Acorn worms inhabit intertidal zones from the White Sea and Greenland to New Zealand and the Cape of Good Hope, and can be found offshore to depths of 400 meters (about 1,300 feet) or more. Pterobranchs are mainly found in the Southern Hemisphere, but some species inhabit northern waters. Acorn worms dwell along the seashore and in water as deep as 3,200 m (10,500 feet), with many living in U-shaped burrows, while some deep-sea species swim freely.
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What are the main types of hemichordates?
Hemichordates are marine invertebrates related to chordates and echinoderms. There are three classes of hemichordates:
* Enteropneusta: Also known as acorn worms, these are solitary, bilaterally symmetrical, wormlike creatures. There are roughly 70 species.
* Pterobranchia: These are minute, colonial, tube-building organisms. Around 20 species have been described.
* Planctosphaeroidea: This class is only known from a few floating larvae.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do hemichordates get their food?
Hemichordates, a group of marine invertebrates, are suspension feeders that capture minute organisms and algae drifting in the water. Acorn worms, a class of hemichordates, trap these organisms on their proboscis, while pterobranchs use their tentacles for the same purpose. All hemichordates use this method to feed. They also have a larval stage, called tornariae, that swim using microscopic hairs called cilia.
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What features make hemichordates different from other invertebrates?
Hemichordates, a group of marine invertebrates, possess unique features that distinguish them from other invertebrates. Unlike chordates, hemichordates lack a tail above the gut and an endostyle that secretes mucus between the gill slits.
Their body is divided into three parts: the proboscis, collar, and trunk. The collar may bear tentacle-like plumes for feeding. They also have a stomochord in the proboscis, once thought to be similar to the chordate notochord. Instead of a central nervous system, nerve tissue concentrates in the collar.
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How are hemichordates related to chordates and echinoderms?
Hemichordates are marine invertebrates that share evolutionary links with both chordates and echinoderms. Molecular studies suggest hemichordates are more closely related to echinoderms than to chordates.
Embryological evidence places the phylum Chordata within the deuterostomes, which also includes the phyla Hemichordata, Echinodermata, and Chaetognatha. The closest relatives of the chordates are probably the hemichordates, since these animals possess gill slits and other features not found in other animal phyla. A slightly more remote relationship to the echinoderms is inferred on the basis of resemblances between the larvae in some groups of hemichordates and echinoderms.
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hemichordate, any of a group of wormlike marine invertebrates closely related to both chordates and echinoderms and usually considered to constitute a phylum, the Hemichordata. The term Hemichordataāfrom the Greek hemi, meaning āhalf,ā and chorde, meaning āstring,ā thus, āhalf-chordateāāwas first proposed because the buccal diverticulum, a tubular outgrowth from the mouth cavity forward into the proboscis, or āsnout,ā resembled a rudimentary notochordāthe dorsal, or back-side, supporting axis of the more primitive vertebrates. This theory has since been rejected, however, because it has been determined that the diverticulum bears little resemblance in origin and function to the vertebrate notochord. Molecular studies indicate that hemichordates are more closely related to echinoderms than to chordates. About 90 species have been described.
The Hemichordata consist of three classes: Enteropneusta, Pterobranchia, and Planctosphaeroidea. Enteropneusts, or acorn worms (about 70 species), are solitary, wormlike, bilaterally symmetrical animals, often brilliantly coloured. They are known as acorn worms because of the appearance of the proboscis and collar. Pterobranchs (about 20 species) are minute, colonial, tube-building forms. Planctosphaeroidea are known only from a few floating larvae. Enteropneusts are common in the intertidal zones from the White Sea and Greenland south to New Zealand and the Cape of Good Hope; they are found offshore to depths of 400 metres (about 1,300 feet) or more. They vary in size from a few centimetres long (Saccoglossus pygmaeus of the North Sea) to two metres (about seven feet) or more (Balanoglossus gigas of Brazilian coastal waters).
The adult hemichordate body and body cavities, or coeloms, are divided into three basic parts: the proboscis, collar, and trunk. There is no central nervous system: nerve tissue is concentrated in the collar, which is linked with a nervous system in the epidermis, or outer covering. The circulatory system usually includes a contractile heartlike vesicle, blood vessels, and sinuses. The pharynx may be perforated by numerous paired gill slits, or they may be absent.
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The second region of the body, the collar, may bear two or more tentacle-like plumes, which may have a double row of ciliated tentacles well supplied with secretory cells. The tentacles are special adaptations for feeding on particles suspended in the water. The network of nerve cells and fibres lying within the epidermis is linked with two main nerve tracts that lie dorsally median (i.e., toward the body midline on the upper side) and ventrally median (on the lower side). The dorsal side of the collar has a neurochord formed by an inpocketing of the epidermis; it may have a central lumen, or cavity, that opens to the exterior anteriorly and posteriorly, or it may have a series of lacunae, or spaces. The neurochord contains large nerve cells, extensions of which reach almost to the tip of the proboscis and into the ventral nerve cord. These cells probably facilitate rapid responses such as abrupt contractions of the anterior trunk when the proboscis is touched. The general body surface is innervated by a primitive receptor system, which consists of scattered sensory cells. There is no well-defined centre of stimuli and responses.
All hemichordates are suspension feeders, capturing minute animals and algae as they drift by in the water. Acorn worms trap such organisms on their proboscis, whereas pterobranchs use their tentacles. Many hemichordates have a larval stage in their life cycle; the larvae, called tornariae, swim using microscopic hairs called cilia.
The hemichordates have evolutionary links with both chordates and echinoderms (e.g., starfish, sea urchins); however, precise relationships are still being actively studied. See also acorn worm; pterobranch.