Tentacle animals

Japanese: 触手動物 - しょくしゅどうぶつ

The name of the phylum Tentaculata in animal taxonomy, and the group of animals that make up this phylum. They are also called tentacula animals. They were once called pseudomolluscs because of their appearance similar to mollusks, but this term is no longer used. A group of animals that have a crest of tentacles at the front end of their bodies and live either attached or cryptically, including the three classes of brachiopods, bryozoans, and brachiopods. Of these, bryozoans are colonial animals.

[Mawatari Shunsuke]

Features

The tentacle crown consists of a disk-like part with an opening in the middle and several to several dozen tentacles growing from it. This structure is not seen in other animal groups. The tentacles of tentacle animals are unique in that they have a long, tubular cavity inside that is an extension of the mesocoel, and are densely covered with cilia on their surface. Other characteristics are as follows:

(1) It has a U-shaped digestive tract and its anus opens dorsally, outside the crown of tentacles.

(2) Both the larvae and adults are divided into three segments, anterior, middle and posterior, each with anterior, middle and posterior coeloms inside.

(3) They secrete chitinous and calcareous structures to protect their bodies.

(4) It has a circulatory system and reproductive ducts, the kidneys.

(5) Cleavage is complete and radial.

(6) The blastopore becomes the mouth, and the anus opens secondarily.

Exceptions to these characteristics include:

(1) Some brachiopods have an anus that opens at the posterior end of the body, or the digestive tract that ends in a blind pouch.

(2) Bryozoan larvae do not show segmentation, and many adults lack anterior segments.

(3) In brachiopods, the secretions form two dorsal and ventral shells, and in bryozoans, they form chambers that serve as exoskeleton, but in brachiopods, they form a tube that is not attached to the body.

(4) Bryozoans lack a circulatory and excretory system.

(5) Spiral cleavage has been observed in some beetles.

(6) In the case of brookworms, the mouth forms as an extension of the blastopore.

[Mawatari Shunsuke]

Classification

The three classes of tentacle animals mentioned above have been separated since ancient times in geological history, and there are many differences in morphology and development, so some have proposed that they should be considered independent phyla. However, the tentacle crown is a completely common trait, and many of the differences are thought to be adaptive traits in terms of their living ecology.

[Mawatari Shunsuke]

system

There are various theories regarding the phylogenetic position of tentacles in the animal kingdom. Until now, they have been considered to be of the trochophora type and included in the protostomes, based on the fact that the blastopore becomes a mouth and that spiral cleavage is observed. However, recent progress in research on stag beetles has provided evidence that the blastopore itself does not become a mouth, that spiral cleavage is probably an abnormal case, that the larvae are closer to the dipleurula type, and that the three-segment, three-coelom structure of the actinotrocha larvae is inherited as is to the adult. Based on this evidence, the currently prevailing theory is that tentacles are closer to deuterostomes such as hemichordates and echinoderms.

[Mawatari Shunsuke]

[References] | Bryozoans | Brachiopods | Brachiopods

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

動物分類学上の1門Tentaculataの名、およびそれを構成する動物群。触手冠動物(かんどうぶつ)ともよばれる。かつては、軟体動物に外観的に類似するとして擬軟体動物とよばれたこともあるが、現在ではこの語は用いない。体の前端に触手冠をもち、付着あるいは潜生生活をする動物群で、ホウキムシ類、コケムシ類、腕足類の3綱を含む。そのうちコケムシ類は群体を形成する動物である。

［馬渡峻輔］

特徴

触手冠は、中央に口の開く盤状部と、そこから生えた数本から数十本の触手よりなる。他の動物群にはみられない構造である。触手動物の触手は、内部に中体腔(ちゅうたいこう)の延長である細長い管状の腔を有し、表面に繊毛を密生する点で特異である。そのほかの特徴を次に示す。

(1)U字形に曲がった消化管をもち、肛門(こうもん)は触手冠の外、背側に開く。

(2)幼生、成体とも前・中・後の3体節に分かれ、それぞれ前・中・後体腔を内部にもつ。

(3)体の保護のためキチン質や石灰質の構造物を分泌する。

(4)循環系を備え、生殖輸管を兼ねた腎管(じんかん)をもつ。

(5)卵割は全割で放射型。

(6)原口が口となり、肛門は二次的に開口する。

　これらの特徴には次のような例外がある。

(1)腕足類の一部には肛門が体の後端に開くもの、あるいは消化管が盲嚢(もうのう)に終わるものもある。

(2)コケムシ類の幼生は体節制を示さず、成体も多くは前体節を欠く。

(3)分泌物は腕足類では背腹2枚の殻に、コケムシ類では虫室となって外骨格の役目を果たすが、ホウキムシ類では体に密着せずに棲管(せいかん)となる。

(4)コケムシ類は循環系と排出器系を欠く。

(5)ホウキムシ類の一部で螺旋(らせん)卵割が観察されている。

(6)ホウキムシ類では原口の延長上に口ができる。

［馬渡峻輔］

分類

触手動物を構成する前述の3綱は、地史的に古くから分かれており、形態・発生上の相違点も多いため、それぞれ独立の門とする説も提唱されている。しかし、触手冠は完全な共通形質であり、相違点も生活生態上の適応形質と考えられるものが少なくない。

［馬渡峻輔］

系統

動物界における触手動物の系統的位置に関しては諸説があり、これまでは、原口が口になること、螺旋卵割がみられることを重視し、幼生をトロコフォラ型とみなして、前口動物に含められてきた。ところが、近年ホウキムシ類の研究が進み、原口そのものは口にならないこと、螺旋卵割はおそらく異常例であること、幼生はディプリュールラ型に近いこと、アクチノトロカ幼生の3体節3体腔制はそのまま成体へ受け継がれること、などが証拠だてられた。現在ではこれらの証拠に従って、触手動物は半索(はんさく)動物や棘皮(きょくひ)動物などの後口動物に近いとする説が有力である。

［馬渡峻輔］

[参照項目] | コケムシ | ホウキムシ | 腕足類

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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