Hemichordate - Hemichordate

A group of marine invertebrates that make up the phylum Hemichordata. It consists of three classes: Enteropneusta, Pterobranchia, and Graptolithina, of which only colonial tube fossils are known. In the past, the Enteropneusta and Pterobranchia were collectively called Pseudochordata, or the former was called Pseudochordata only, but this term is no longer used. The body of the worm is generally elongated from front to back, and is divided into three parts: the anterior body (snout, head plate), the middle body (collar, neck), and the posterior body (trunk), with the posterior body taking up the majority of the body. The anterior body contains only one body cavity, and the middle body and posterior body each contain a pair of coeloms on the left and right. The midbody is a short hollow cylinder, with the posterior end of the anterior body connected to the inner wall of the midbody. The mouth is bounded by the ventral surface of the anterior body and the anterior edge of the midbody. In enterobranchiata, the digestive tract is thick and straight through the center of the posterior body and opens at its posterior end, but in pterobranchia, it bends dorsally in a U-shape inside the posterior body. Only in pterobranchia do one or more pairs of tentacle arms protrude from the midbody. The organ (oral blind tube), which is formed when the dorsal median part of the midbody wall invaginates into the center of the anterior body in an elongated cylindrical shape, was once considered to be homologous to the notochord, but today this is strongly opposed. Rather, the presence of gill slits in the pharyngeal wall, except in some pterobranchia, is the basis for the close relationship between hemichordates and notochordates. It is particularly noteworthy that the structure of the gill slits in enterobranchiata is very similar to that of cephalochordates, but hemichordates do not have any peribranchial cavity, which is commonly seen in protochordates. The body surface is covered with a simple ciliated epithelium and is rich in mucus. Nerve cells and nerve fibers are located at the base of the epidermal layer. The nervous center has not been identified. The vascular system is open, and colorless blood containing blood cells is circulated by contraction of the pericytes (located in the anterior body), which are closed sacs completely independent of the blood vessels. Excretion is thought to be carried out by an organ called the pericardium (located in the anterior body).

Enterobranchiata are always solitary and usually live free, hiding in the bottom sediment. On the other hand, most of the pterobranchs live in colonies, either in a tube secreted by themselves or connected to each other at the rear end of the posterior part of the posterior body, and catch suspended organic matter in the water with their tentacle arms. Hemichordates are dioecious, and enterobranchiata can undergo indirect development, which produces a floating larva called tornaria, which is said to be very similar to the larvae of some echinoderms, as well as direct development, which does not produce tornaria. In pterobranchs, in addition to direct development through a planula-type larva, indirect development is also thought to occur, but this is still uncertain. Asexual reproduction by budding is common in pterobranchs, and budding and regeneration are known in enterobranchiata as well. There are currently 33 species of pterobranchs in 2 families, 8 genera, and they are distributed all over the world, including the polar regions, from the intertidal zone to deep seas of 1,500 meters. In addition to Enokorofusakatsutsugi, one other species has been recorded from the waters off Japan.

The taxonomic position of graptolites has been much debated, but since the structure of their burrows is very similar to that of some pteridobranchs, it is generally considered to be a member of the Hemichordata, but there are also dissenting opinions, such as the Cnidaria theory. They lived from the middle Cambrian period to the Carboniferous period of the Paleozoic era, and flourished especially in the Ordovician and Silurian periods. They have been excavated widely all over the world, and are useful for comparing geological strata. In Japan, they have been discovered in Silurian limestone in Shikoku in recent years.

[Teruaki Nishikawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

動物分類学上、一門Hemichordataを構成する海産無脊椎(せきつい)動物の一群。腸鰓綱(ちょうさいこう)（ギボシムシ類）Enteropneusta、翼鰓綱(よくさいこう)Pterobranchia、および群体性の棲管化石(せいかんかせき)のみが知られる筆石綱(ふでいしこう)Graptolithinaの3綱からなる。かつては、腸鰓類および翼鰓類を含め、または前者のみに対して擬索類(ぎさくるい)と称したが、現在は用いない。虫体は一般に前後に細長く、前体（吻(ふん)、頭盤(とうばん)）、中体（襟(えり)、頸(くび)）、後体（躯幹(くかん)）に三分され、後体が大部分を占める。前体はただ一つの、そして中体と後体は左右に対(つい)をなす体腔(たいこう)をそれぞれ1対含む。中体は中空の短い円筒形で、その内壁背面に前体の後端部が連結している。前体腹面と中体前縁とで囲まれたところが口にあたる。消化管は、腸鰓類では後体の中心部を太くまっすぐに貫通してその後端に開くが、翼鰓類では後体の中でU字形に背方に曲がる。翼鰓類に限り、中体から1対またはそれ以上の触手腕が突出している。中体内壁の背正中部が前体の中心部に長円筒形に陥入してできた器官（口盲管）が、かつては脊索(せきさく)と相同とみなされたが、現在では否定的見解が強い。むしろ、翼鰓類の一部を除き咽頭(いんとう)壁に鰓裂がみられることが、半索動物と脊索動物との近縁性の根拠となっている。とくに腸鰓類の鰓裂の構造は頭索類のそれと酷似することは注目されるが、半索動物には原索動物一般にみられる囲鰓腔はまったくない。体表は単層繊毛上皮で覆われて粘液に富む。神経細胞や神経繊維は表皮層の基部にある。神経中枢は確定されていない。血管系は開放的で、無色の血液は血球を含み、血管から完全に独立した閉じた袋である心胞（前体にある）の収縮により循環する。排出は、前体にある脈毬(みゃくきゅう)という器官が行うと考えられている。

　腸鰓類はつねに単独で、普通、底質中に潜んで自由生活する。一方、翼鰓類ではその大部分が自ら分泌した棲管の中に集合、もしくは後体後端で互いに連結した群体ですみ、触手腕で水中の懸濁有機物をとらえる。半索動物は雌雄異体で、腸鰓類では、棘皮(きょくひ)動物のある種の幼生のものとよく似ているとされる浮遊幼生トルナリアを出す間接発生のほか、これを出さない直接発生もある。翼鰓類ではプラヌラ型幼生を経る直接発生のほか、間接発生もあると考えられているが、現在はまだ不確実である。翼鰓類では出芽による無性生殖が盛んで、腸鰓類にも出芽や再生が知られている。翼鰓類にはこれまで2科8属33種が知られ、極地方を含む全世界の潮間帯から1500メートルの深海にまで分布する。日本近海からはエノコロフサカツギのほか1種が記録されている。

　筆石類の分類学的位置はさまざまに議論されてきたが、翼鰓類のあるものと棲管の構造がよく似ていることから、現在では半索動物の一員とする見解が一般的であるが、刺胞動物説などの異論もある。古生代カンブリア紀中期から石炭紀まで生存し、とくにオルドビス、シルル両紀に大繁栄した。広く全世界に出土し、地層の対比に役だっている。日本でも近年、四国のシルル系石灰岩から発見された。

［西川輝昭］

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