Brachiopods - Wansokurui

A group of animals that make up the class Brachiopoda in the phylum Tentacula. All of them live in the sea and are divided into two main groups: the Brachiopoda (subclass Inarticulate) and the Physalis (subclass Articulate). They have two shells and a stalk that comes out from the base of the shell, and live either burrowing into the sand and mud on the seabed or attached to rocks. They are often mistaken for bivalve mollusks because of their similar appearance, but they can be distinguished by the fact that their two shells are located on the dorsal and ventral sides of the body and are shaped differently. Most species have a shell length of around a few centimeters.

The shell contains the soft body filled with internal organs, a membranous mantle that secretes the shell, and a crown of tentacles. Compared to the other two classes of Tentacula (Bryozoa and Brachiopods), the crown of tentacles of brachiopods is particularly well developed and large, and is specifically called an arm. To support this arm, a structure called the arm bone is developed on the inside of the dorsal shell. The shape of this arm bone is important for species identification, especially in the case of fossil species.

The digestive tract is U-shaped, and the anus opens to the outside of the crown of tentacles. Nightshade species do not have an anus. They have a closed vascular system. They have a pair of nephric ducts that serve as both excretory and reproductive ducts. They are dioecious. Nightshade larvae have two shells, float for a while, then settle to the bottom and become adults without metamorphosing. Nightshade larvae do not have shells, settle to the bottom about a day after hatching, and metamorphose into adults.

Brachiopods are an ancient group of animals that appeared in the Cambrian Period of the Paleozoic Era. They are an important group of animals in paleontology and geology because a large number of fossils with distinctive characteristics are produced in each geological period. In taxonomy, this group of animals is sometimes elevated to the level of a phylum.

[Mawatari Shunsuke]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

触手動物門の1綱Brachiopodaを構成する動物群。すべて海産で、シャミセンガイの仲間（無関節亜綱）とホオズキガイの仲間（有関節亜綱）とに二大別される。2枚の殻と、殻の根元から出る肉茎とよばれる柄をもち、海底の砂泥中に潜るか、岩などに付着してすむ。外見上、軟体動物の二枚貝類とよく似ているので間違われるが、2枚の殻は体の背腹に位置し、互いに形が異なっていることで区別できる。殻長数センチメートルくらいの種が多い。

　殻の中には、内臓の詰まった軟体部、殻を分泌する膜状の外套(がいとう)、そして触手冠が収まっている。触手動物門のほかの2綱（コケムシ類とホウキムシ類）に比べて、腕足類の触手冠はとくによく発達していて大きく、特別に腕(うで)とよばれる。この腕を支えるために、背側の殻の内側には腕骨(わんこつ)とよばれる構造が発達している。種の同定には、化石種の場合はとくに、この腕骨の形が重要となる。

　消化管はU字形で、肛門(こうもん)は触手冠の外側に開く。ホオズキガイの仲間は肛門をもたない。血管系は閉鎖型。排出器と生殖輸管を兼ねた1対の腎管(じんかん)をもつ。雌雄異体。シャミセンガイ類の幼生は2枚の殻をもち、しばらく浮遊したのちに着底して変態せずにそのまま成体となる。ホオズキガイ類の幼生は殻をもたず、孵化(ふか)後1日ほどで着底し、成体へと変態する。

　腕足類は、古生代カンブリア紀に出現した古い動物群である。各地質時代ごとに、それぞれ特徴的な多種多数の化石が産出するため、古生物学や地質学においても重要な動物群である。なお、分類学上、この動物群を門の段階に昇格させて扱うこともある。

［馬渡峻輔］

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