Molluscs - Molluscs

A general term for animals that make up the phylum Mollusca, a taxonomic division of animals. On Earth today, they are the second largest group of animals after arthropods. With a few exceptions, their bodies are basically flexible, have no obvious segments, and are protected from the back by a shell. They consist of three parts: the head, visceral mass, and feet. The head has antennae and eyes, and the mouth has a radula. Between the visceral sac and the mantle is the mantle cavity, where gills are found. The mantle secretes a shell, and the feet are large and adapted for crawling.

There are about 110,000 extant species of mollusks in the world, which are divided into the following seven classes: (1) Aplacophora: These are marine mollusks with worm-like bodies lacking shells, with the dorsal side covered in a cuticle layer and bone fragments embedded inside. They consist of two groups: the Cleft Gastropods, which have fine grooves on the front and back of the ventral side and are typified by the Casemida, and the Caudata, which have no grooves and are typified by the Polyplacophora. No fossils have been found, and there are about 200 extant species in the world and about 15 in Japan. (2) Polyplacophora: These are marine mollusks, also known as the Chitons. They have oval bodies with eight shells arranged front to back. They appeared in the late Cambrian period of the Paleozoic era, and there are about 1,000 extant species in the world and about 90 in Japan. (3) Monoplacophora: Most of these mollusks are deep-sea mollusks, typified by the Neopirinae. A type of shellfish that appeared in the early Cambrian period of the Paleozoic era and flourished until the Devonian period. There are 11 extant species in the world, but none are found in the seas near Japan. (4) Gastropoda: These are the so-called snail shells that live in the sea, in fresh water, and on land, and have shells that are usually spirally wound. They have a pair of antennae and eyes, and their feet are large and wide, making them suitable for crawling. They appeared in the early Cambrian period of the Paleozoic era, and there are 85,000 extant species in the world, and about 4,500 species in Japan. (5) Scisopoda: These are marine. They have tubular shells that bend like a bow. They have no eyes or antennae on their heads. They appeared in the Devonian period of the Paleozoic era, and there are 500 extant species in the world, and about 70 species in Japan. (6) Bivalves: These are the so-called bivalves that live in the sea and fresh water, and their bodies are surrounded by two shells on both sides. They lack a head, have wedge-shaped legs and two pairs of gills, and in advanced species, the rear of the mantle has two inlet and outlet canals. They appeared in the early Ordovician period of the Paleozoic era, and there are currently 25,000 species in the world, and about 1,500 species in Japan. (7) Cephalopoda - Marine. They are squid and octopus-like creatures, and their body has the torso, head, and legs aligned in a straight line. This is a major difference from the general structure of the phylum Mollusca, which carries its visceral mass on the dorsal side of the body and has its legs on the ventral side. The shell is generally degenerate, but primitive nautiluses have a rolled exoskeleton. They appeared in the Cambrian period of the Paleozoic era, and there are currently about 650 species in the world, and about 150 to 200 species in Japan.

Molluscs are not only an important source of food for humans, but also serve as raw materials for crafts and medicines, and are also vectors of parasites and pests in fishing, making them an animal group that is extremely closely related to human life.

[Takashi Okutani]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

動物分類学上の一門である軟体動物門Molluscaを構成する動物の総称。現在の地球上では、節足動物に次いで大きな動物群である。体は、多少の例外はあるが基本的には柔軟で、明らかな環節がなく、背面から殻で保護されている。頭部、内臓塊、足部の3部分からなり、頭には触角や目があり、口の中には歯舌がある。また、内臓嚢(のう)と外套(がいとう)との間は外套腔(こう)で、ここにえらがある。外套は殻を分泌し、足は大きくて、はうのに適している。

　軟体動物には世界に約11万程度の現生種があり、次の7綱に分けられている。(1)無板綱　海産で、体はミミズ状で殻を欠き、背側はクチクラ層に覆われ、中に骨片が埋もれている。腹側に前後に細かい溝がありカセミミズ類が代表的な溝腹類と、溝のないケハダウミヒモ類が代表的な尾腔類の2群からなる。化石は未発見で、現生種は世界に約200種、日本には約15種が知られている。(2)多板綱　海産で、ヒザラガイ類ともよばれる。体は楕円(だえん)形で、前後に8枚の殻が並ぶ。古生代カンブリア紀後期から出現し、現生種は世界に約1000種、日本には約90種を産する。(3)単板綱　大部分が深海産で、ネオピリナ類で代表される。古生代カンブリア紀初期から出現し、デボン紀まで栄えた類で、現生種は世界に11種、日本近海には産しない。(4)腹足綱　海中、淡水中、陸上にすみ、殻は普通螺旋(らせん)状に巻いているいわゆる巻き貝類である。1対の触角と目があり、また、足が大きくて広く、はうのに適している。古生代カンブリア紀初期から出現し、現生種は世界に8万5000種、日本に約4500種を産する。(5)掘足綱　海産。殻は管状で弓状に曲がるいわゆるツノガイ類。頭には目も触角もない。古生代デボン紀から出現し、現生種は世界に500種、日本に約70種を産する。(6)二枚貝綱　海中、淡水中に産し、体を2枚の殻で左右から囲む。頭を欠き、足はくさび形でえらは2対あり、進んだ仲間では外套膜の後方は2本の出入水管になる。古生代オルドビス紀初期に出現し、現生種は世界に2万5000種、日本に約1500種を産する。(7)頭足綱　海産。イカ・タコ類で、体は胴部、頭部、足部が一直線に並んでいる。この点が、体の背側に内臓塊を背負い、腹側に足のある軟体動物門一般の体制と大きく異なっている。殻は一般に退化的であるが、原始的なオウムガイでは巻いた外殻がある。古生代カンブリア紀に出現し、現生種は世界に約650種、日本には約150～200種を産する。

　軟体動物は、人類にとって重要な食料となるばかりでなく、工芸材料、薬品などの原料となる一方、寄生虫の媒介者、漁業上の害敵となるなど、きわめて人間生活と関係の深い動物群であるといえる。

［奥谷喬司］

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