コケ動物無脊椎動物

モスの動物とも呼ばれ、コケムシ、約5,000種の現存する種であるコケムシ門（PolyzoaまたはEctoproctaとも呼ばれる）のメンバー。別の15,000種は化石からのみ知られています。腕足類とホロ目と同様に、コケムシは、水中に浮遊している食物粒子を収集するために、ロフォフォアと呼ばれる繊毛触手の独特の輪を持っています。コケムシは広く分布している水生無脊椎動物の動物のグループであり、そのメンバーはズーイドと呼ばれる多数の接続されたユニットで構成されるコロニーを形成します（したがって、「多くの動物」を意味する用語Polyzoa）。一部の種のコロニーは直径が0.5メートル（約20インチ）を超える可能性がありますが、個々の動物園は通常1ミリメートル（0.04インチ）以下の長さです。 18世紀半ばまで、コケムシはサンゴと同様に植物と見なされていました。したがって、「苔の動物」を意味する名前です。 75年後、コケムシは刺胞動物とは区別されており、まず、動物の特徴的な構造について説明しました。

コケムシは、3つのクラスに分類されます：Phylactolaemata（淡水住居）; Stenolaemata（海洋）; そしてGymnolaemata（主に海洋）。600属を含むCheilostomata（Gymnolaemataクラス）の注文は、最も成功したコケムシのグループです。

一般的な機能

分布と豊富さ

Bryozoan colonies are found in both fresh and salt waters, most commonly as growths or crusts on other objects. Freshwater bryozoans live among vegetation in clear, quiet, or slowly flowing water. Marine species range from the shore to the ocean depths but are most plentiful in the shallow waters of the continental shelf. They cover seaweeds, form crusts on stones and shells, hang from boulders, or rise from the seabed. Bryozoans readily colonize submerged surfaces, including the hulls of ships and the insides of water pipes. A few types of bryozoans form nonattached populations on sandy seabeds.

The zooid walls, which constitute the most permanent portion of the colony, generally are calcareous (i.e., impregnated with calcium carbonate), giving bryozoans a fossil record that dates from the Ordovician onward (i.e., from about 500 million years ago).

Size range and diversity of structure

Although the component zooids rarely exceed one millimetre in length, bryozoan colonies—formed of numerous asexually budded zooids—vary greatly in size. In the gymnolaemate genus Monobryozoon, which lives between marine sand particles, a colony consists of little more than a single feeding zooid less than one millimetre in height. Colonies of the European Pentapora, however, can reach one metre (3.3 feet) or more in circumference; a warm-water gymnolaemate genus, Zoobotryon, which hangs from harbour pilings, and the freshwater phylactolaemate Pectinatella each produce masses that may be one-half metre across. Colonies that form crusts generally cover only a few square centimetres; erect colonies may rise only two to five centimetres (0.8–2 inches).

The texture of the colonies is variable. Some colonies, especially those in fresh water and on seashores, are gelatinous or membranous; others are tufted, with flat fronds (leaflike structures) or whorls of slender branches, whose horny texture results from light deposits of lime in zooid walls. Still other colonies are hard and have calcified skeletons. Such colonies may form rough-surfaced patches or may rise in slender branching twigs (such as those that form a network in the beautiful lace corals; e.g., Sertella).

The colonies, diverse and complex in structure, are composed of individual modules, or zooids, and each zooid effectively is a complete animal. In all bryozoan colonies, however, the zooids remain interconnected and may exchange nutrients and other substances through interconnecting cables or minute pores in their body walls. A bryozoan colony usually has many zooids, which may be of one type or of types that differ both functionally and structurally. All zooids in a colony arise by asexual budding from the first zooid to form. Zooids capable of feeding have a ring of slender tentacles at one end of the body. Cilia (hairlike projections) that propel tiny particles of food toward the zooid mouth are found on this ring, and the whole feeding organ is called a lophophore. The mouth opens into a digestive tract that is divided into several regions and terminates at an anus, which is outside (but near) the tentacles (hence the name Ectoprocta, meaning “outside anus”). If zooids are disturbed, they withdraw their tentacles inside the body cavity. Only if the zooids have transparent walls, such as in the gymnolaemates Bowerbankia and Membranipora, is the digestive tract visible. The internal living parts of each zooid—i.e., the nervous and muscular systems, the tentacles, and the digestive tract—are called the polypide.

Natural history

Reproduction and life cycle

Many animals, bryozoans included, have a life cycle that incorporates phases of asexual and sexual reproduction. Asexual reproduction, in which no gametes (sex cells) participate, produces genetically identical progeny (clones), which separate in larger animals (e.g., sea anemones). In bryozoans, the progeny, called zooids, are produced by an asexual process called budding and almost invariably remain in intimate contact to form a colony. As the colony continues to enlarge by budding, the zooids become sexually mature, producing eggs and spermatozoa. Sexual reproduction, by the production and subsequent fusion of gametes, generates the genetic variability necessary for a species to adapt to changing conditions. Fertilized eggs develop into swimming larvae.

Some bryozoans also propagate colonies asexually. The cheilostome Discoporella has small, nonattached, saucerlike colonies. Groups of zooids at the colony rim detach at special fracture zones and grow into new colonies. The statoblasts (dormant buds) of freshwater bryozoans are another asexual means of reproduction. Asexual reproduction, whether leading to a clone, a colony, or a clone of colonies, is a means of perpetuating and spreading a successful genetic constitution (genotype).