Bivalve mollusks - Nimai Ii

A class in the phylum Mollusca. Many of the bivalve mollusks in this class are also called axe-shaped axe-footed mollusks, and are also called valvular branchial mollusks. Usually, the bilaterally symmetrical shell encases the soft body from both sides. The two shell pieces are connected by ligaments and interlock with teeth on a hinge. The mantle membrane covers the entire soft body, like a backing for the shell, and in higher groups, the left and right mantle membranes are fused at the rear of the body to form an inlet and outlet canal. The left and right shells are closed by adductor muscles at the front and back. Two pairs of broad leaf-like gills hang from the mantle cavity, which filter organic matter from the water they breathe and use as food. They lack what is called a head, and therefore have no head antennae or eyes, and are the only exception among all mollusks in that they lack a radula in their mouth, but they do have a lip for selecting food. Their stomachs usually contain rods (gelatinous rods that secrete digestive enzymes and twist food that enters the mouth into a string-like shape to draw it into the digestive tract), and most have intestines that penetrate the ventricle. In classification, the symmetry of the shell, the general shape, and the shape of the dentition are used as important indicators, and there are various types that take into account factors such as the shape of the gills.

Many bivalve species are marine, with only a few being freshwater, but broadly speaking, they can be divided ecologically into epiphytic ones (mostly pteromorphs) that are attached to the hard bottom by means of byssus, and endophytic ones that live underground in the soft bottom. Most bivalve mollusks are dioecious, but some are known to be hermaphroditic and undergo sex change. Most species are oviparous, and fertilization usually takes place in the sea, but some species are viviparous, and the offspring are raised in the gill chambers of the mother mollusk until they hatch.

[Takashi Okutani]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

軟体動物門の1綱。この二枚貝綱Bivalviaは、多くのもので足が斧(おの)形をしているため斧足類(おのあしるい)の別名があり、弁鰓類(べんさいるい)ともいう。通常、左右相称の貝殻で軟体を左右から包む。両殻片は靭帯(じんたい)で連結され、また蝶番(ちょうつがい)（鉸歯(こうし)）の歯でかみ合う。外套(がいとう)膜は貝殻を裏打ちするようなかっこうで軟体全体を覆い、高等なグループでは体後方で左右の外套膜が癒合して出・入水管を形づくっている。左右の殻は前後にある閉殻筋によって閉められている。外套腔(こう)には広い葉状のえらが2対垂れ下がっていて、これで呼吸水中の有機物を漉(こ)し取り食物とする。頭部とよばれる部分を欠き、したがって頭部触角、目はなく、軟体動物全綱のうち唯一の例外として口には歯舌を欠くが、餌(えさ)を選別する唇弁がある。胃には通常、桿晶体(かんしょうたい)（消化酵素を分泌すると同時に、口から入った餌を紐(ひも)状にねじり消化管に引き入れる機能を果たすゼラチン状の棒）があり、腸は心室を貫くものが多い。分類には、殻の相称性、概形、鉸歯の形態が重要な標徴として用いられるほか、えらの形式などが加味されるなど種々の形式がある。

　二枚貝類には海産種が多く、淡水産のものは一部であるが、生態的に大別すれば、硬底上に足糸などで固着生活する表生性のもの（多くは翼形類）と、軟底中に潜入生活する内生性のものに分けられる。二枚貝綱の多くは雌雄異体であるが、雌雄同体のものや、性転換するものが知られている。卵生の種が多く、受精は多くは海中で行われるが、一部の種は胎生で、母貝の鰓室内などで子貝になるまで哺育(ほいく)される。

［奥谷喬司］

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