腕足类

腕足类 brākēŏp´ədə[关键］有壳无柄或定居的海洋动物门，通常被称为灯壳，特点是有一个特殊的进食器官，管胞。外壳由两部分组成，称为阀门，它们完全包围着身体;这种动物的外表很像双壳类软体动物或足足类动物，如蛤蜊。然而，灯壳的瓣膜覆盖动物的顶部和底部，而蛤壳的瓣膜覆盖左右两侧。此外，腕足类动物的内部解剖结构与足足类动物不同;这两组人没有关系。在门中有两类:无节肢动物，其中的瓣膜仅靠肌肉连接在一起;有节肢动物，其中的节肢动物有相互连接的突起，形成铰链。一组复杂的肌肉打开外壳进行捕食，关闭外壳进行保护。在大多数腕足动物中，一种叫做花梗的短柄在瓣膜之间或通过下瓣膜的开口露出来。大多数无梗腕足动物通过花梗附着在物体上，但少数没有花梗，直接通过腹侧瓣膜附着。 Burrowing lamp shells have long pedicels, which they contract to retreat into the burrow. The lophophore consists of two tentacle-bearing arms, often spirally coiled, one on either side of the mouth. The tentacles have cilia that create currents, drawing water-bearing food particles and oxygen into the shell. Food particles are trapped in mucus on the tentacles and moved by the cilia to the mouth. Oxygen is absorbed through the body wall. Brachiopods have a simple digestive and nervous system, and are equipped with excretory organs called nephridia. The open circulatory system includes a contractile vessel, or heart, and sinuses for the flow of the colorless circulatory fluid to various parts of the body. Reproduction is sexual and the sexes are usually separate. In most species the eggs and sperm are shed into the sea, where fertilization results in the development of free-living, ciliated larvae. The larvae settle to the bottom after developing rudiments of the adult structures. A few species brood their young. Brachiopods are believed to be related to the shell-less bryozoans, or moss animals (phylumEctoprocta)，它们也有一个孔胞。腕足动物在寒武纪初期数量丰富，在古代海洋中分布广泛，数量众多。已知的灭绝物种约有12000种，其中最大的物种直径近1英尺(30厘米)。如今现存的物种不足325种，而且数量相对较小，通常为1到2英寸。(直径2.5-5厘米)。它们都是海生的，大多数喜欢浅水;它们零星分布，虽然有些在当地非常丰富。在比较著名的灯壳中是穴居灯壳海豆芽(无茎纲)和无柄脑壳(类关节动物)。

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