Protochordate - Protochordate

A general term for marine animals that have a notochord, a hollow neural tube located behind it, gill slits (a slit in the lateral wall of the pharynx), and an endostyle running vertically down the ventral midline of the pharynx at least for a certain period of their life cycle, but do not have a vertebrate throughout their life. They are broadly divided into those with a notochord at the tail (Urochordata, or Tunicata) and those with a notochord over the entire length of their body (Cephalochordata). Of the characteristics listed at the beginning, the endostyle is considered to be the precursor of the thyroid gland in vertebrates, and all other characteristics except for the vertebrate are common to vertebrates. For this reason, there is a recent tendency to consider the three animal groups, urochordates, cephalochordata, and vertebrates, as closely related in systemic order and to place them in three subphyla of the phylum Chordata, but the system of separating the first two groups as Protochordata is still in use. In the past, hemichordates were also considered to be part of the protochordates, but the homology between the oral blind canal of the former and the notochord of the latter has been denied, and they are now excluded from the protochordates. In recent years, fossils of animals that contain microfossil conodonts have been found, and it has been proposed to consider these as chordates and establish the subphylum Conodontochordata, or to consider the fruits (fossils) that were previously classified as echinoderms as the ancestral form of all chordates and establish the subphylum Calcichordata, but the former is widely disagreed with, and the latter has not yet gained widespread support.

The structure of the notochord is extremely diverse, which has led to various discussions regarding the phylogenetic evolution of protochordates. In cephalochordates, the notochord is muscular and controlled by the spinal nerves. On the other hand, in urochordates, some ascidian larvae and tail worms have a structure in which a flattened simple epithelium encases the extracellular matrix, but in ascidian and sea urticate larvae, the notochord is made up of cells arranged in a row. In vertebrates, the notochord is generally made up of many vacuolated cells.

[Teruaki Nishikawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

脊索(せきさく)およびその背方に位置する中空の神経管、鰓裂(さいれつ)（咽頭(いんとう)側壁の裂け目）、それに咽頭腹正中を縦走する内柱を、生活史の少なくとも一時期にもつが、脊椎(せきつい)は終生もたない海産動物の総称。脊索を尾部にもつもの（尾索類Urochordata＝被嚢(ひのう)類Tunicata）と、それが体全長にわたるもの（頭索類Cephalochordata）とに大別される。冒頭にあげた諸特徴のうち、内柱は脊椎動物の甲状腺(せん)の先駆とみなされ、また脊椎を除くほかの特徴は脊椎動物と共通する。このため、尾索、頭索、脊椎の3動物群を系統的に近縁とみなして、脊索動物門Chordataの3亜門にそれぞれ位置づける近年の傾向があるが、前二者を原索動物門Protochordataとして独立させる体系もなお用いられている。かつては、半索動物をも原索動物の一員とみなしたが、前者の口盲管と後者の脊索との相同性が否定され、現在では原索動物から除外している。なお、近年、微化石コノドントを体内にもった動物の化石が得られ、これを脊索動物とみなして錐歯索動物亜門(すいしさくどうぶつあもん)Conodontochordataをたてる説、あるいは従来棘皮(きょくひ)動物に位置づけられてきた海果類（化石）を、すべての脊索動物の祖先形とみなしてカルキコルダータ亜門Calcichordataをたてる説が提唱されているが、前者は異論が多く、後者もまだ大方の支持を得るに至っていない。

　脊索の構造には多様性が著しく、原索動物の系統進化に関する種々の議論をよんでいる。すなわち、頭索類では、脊索は筋肉性で脊髄神経の支配を受ける。一方、尾索類においては、ホヤ類幼生のあるものや尾虫類では、細胞外基質を扁平(へんぺい)単層上皮が包み込む構造であるが、マボヤやウミタル類の幼生では、一列に配列した細胞からなる。なお、脊椎動物では一般に多数の空胞細胞から構成される。

［西川輝昭］

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