Characteristics of Hemichordata:- 2 Body has more than two cell layers, tissues and organs. The Hemichordates, or Acorn Worms are a small phylum of generally small animals that were once considered to be a part of the Chordata. However modern science has proven that none of them posses a post anal tail or a notochord and they have been allocated to a phylum of their own. They are all marine worm-like animals.
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Among the phylum Hemichordata are the enteropneusts acornworms , which are worm-shaped inhabitants of shallow seas and have a short, conical… 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 metres about 1, 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. 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. 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. Get exclusive access to content from our First Edition with your subscription.
Subscribe today 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. See also acorn worm ; pterobranch. This article was most recently revised and updated by John P. Rafferty , Editor.
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Introduction to the Hemichordata Of sawblades and acorns. Not familiar creatures to most people, hemichordates form a small phylum only a few hundred species. Their importance for the study of vertebrate evolution, however, cannot be underestimated. The fossil record of one group of hemichordates, the graptolites, is very well known and is often used to correlate rocks. Hemichordates are distinguished by a tripartite threefold division of the body. At the forward end of the body is a preoral lobe, behind this is a collar, and last comes a trunk.
Introduction to the Hemichordata
Rhabdopleura Single genus. Class 3. Planctosphaeroidea: This class is represented by a few small rounded, transparent and pelagic larvae, supposed to be specialised tornaria of some unknown hemichordate Planctosphaera pelagica. The larva has branching arborescent ciliated bands on the surface.
The Phylum Hemichordata
Anatomy[ edit ] The body plan of hemichordates is characterized by a muscular organization. The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome. The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles. The mouth is located between the proboscis and the collar. The trunk is the longest part of the animal.