HEMICHORDATE

any of a group of wormlike marine invertebrates closely related to the chordates and sometimes considered to constitute a phylum, Hemichordata. The term Hemichordata-from the Greek hemi, meaning "half," and chorde, meaning "string," thus, "half-chordate"-was first proposed because the buccal diverticulum, a tubular outgrowth from the mouth cavity forward into the proboscis, or "snout," resembled a rudimentary notochord-the 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. Although the hemichordates are sometimes given phylum rank, some authorities place the group at various other taxonomic levels with the other so-called protochordates. The Hemichordata consist of three classes: Enteropneusta, Pterobranchia, and Planctosphaeroidea. Enteropneusta, or acorn worms, are solitary, wormlike, bilaterally symmetrical animals, often brilliantly coloured. They are known as acorn worms because of the appearance of the proboscis and collar. Pterobranchia 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 is distinguished by the division of its body and body cavities, or coeloms, into three basic parts: the proboscis, collar, and trunk. A central nervous system is absent, but there is a concentration of nerve tissue 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.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 filter feeders, extracting minute animals and plants that drift in the water. Acorn worms trap such organisms on their proboscis, whereas pterobranchs use their tentacles. The Planctosphaera larvae, which occur on the ocean surface, trap the microscopic organisms in currents created by the movement of bands of tiny hairlike processes called cilia. Many hemichordates have a larval stage in their life cycle; the larva is called a tornaria. The hemichordates have remained a general primitive group, at a low level of evolution, closely linked with the echinoderms (e.g., starfish, sea urchins) and sharing with them and the other protochordate groups a sessile (i.e., fixed to a surface) or semi-sessile ancestor, which was bilaterally symmetrical and had a tripartite body and coelom. The comparatively simple larval development of the Hemichordata suggests that they have deviated less from the ancestral stock than have echinoderms or protochordates. See also acorn worm; pterobranch.