主題:動物/精選條目/4

內肛動物門（學名：Entoprocta）是動物界的一個門。分類上曾一度把牠們與外肛動物門合稱為苔蘚動物門（Bryozoa）。後來發現內肛動物為假體腔動物，而外肛動物為真體腔動物，現行分類已將這兩類動物獨立成門。內肛動物大約有150多種，絕大多數體型不超過5mm。 內肛動物門，水生無脊椎動物，分佈於全世界，主要見於海洋生境。有少數生活於淡水。內肛動物為單體或通過出芽方式形成相互交通的群體；其中，每個個體稱為個員，高僅0.4～0.5毫米，有一條柄，可以互相結合，柄可附着在貝殼、海草或其他會產生水流的動物，如海綿、苔蘚動物、水螅和環節動物上。身上有一個球狀的頭部（萼），其中包容一個U形的消化道以及神經、排泄和生殖系統。具纖毛的觸手排成冠狀，用以產生水流，將食物顆粒送入位於中央的口部。營群體生活的各類常將萼總拋棄，然後再生。營單體生活的種類可以生出子芽，子芽成熟後脫離母體。雖然內肛動物的幼體及其早期發育方式表明內肛動物可能與環節動物和與環節動物近緣的門有遙遠的親緣關係，但無證據表明內肛動物與任何門的動物為近緣。

Entoprocta is a phylum of mostly sessile aquatic animals, ranging from 0.1 to 7 millimetres (0.0039 to 0.28 in) long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that draw food particles towards the mouth, and both the mouth and anus lie inside the "crown". The superficially similar Bryozoa (Ectoprocta) have the anus outside a "crown" of hollow tentacles. Most families of entoprocts are colonial, and all but 2 of the 150 species are marine. A few solitary species can move slowly. Some species eject unfertilized ova into the water while others keep their ova in brood chambers until they hatch, and some of these species use placenta-like organs to nourish the developing eggs. After hatching, the larvae swim for a short time and then settle on a surface. There they metamorphose, and the larval gut generally rotates by up to 180°, so that the mouth and anus face upwards. Both colonial and solitary species also reproduce by cloning — solitary species grow clones in the space between the tentacles and then release them when developed, while colonial ones produce new members from the stalks or from corridor-like stolons. Some species of nudibranchs ("sea slugs") and turbellarian flatworms prey on entoprocts. A few entoproct species have been found living in close association with other animals. It is uncertain whether any are invasive species.