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In biological anatomy, commonly referred to as the mouth, under formal names such as the oral cavity, buccal cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing in higher vertebrates the tongue and teeth. This cavity is also known as the buccal cavity, from the Latin ''bucca'' ("cheek"). Some animal phyla, including vertebrates, have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostome and deuterostome. ==Development== In the first multicellular animals there was probably no mouth or gut and food particles were engulfed by the cells on the exterior surface by a process known as endocytosis. The particles became enclosed in vacuoles into which enzymes were secreted and digestion took place intracellularly. The digestive products were absorbed into the cytoplasm and diffused into other cells. This form of digestion is used nowadays by simple organisms such as ''Amoeba'' and ''Paramecium'' and also by sponges which, despite their large size, have no mouth or gut and capture their food by endocytosis. The vast majority of other multicellular organisms have a mouth and a gut, the lining of which is continuous with the epithelial cells on the surface of the body. A few animals which live parasitically originally had guts but have secondarily lost these structures. The original gut of multicellular organisms probably consisted of a simple sac with a single opening, the mouth. Many modern invertebrates have such a system, food being ingested through the mouth, partially broken down by enzymes secreted in the gut, and the resulting particles engulfed by other cells in the gut lining. Indigestible waste is ejected through the mouth.〔 In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the formation of the gut. In deuterostomes, the blastopore becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth (''proto–'' meaning "first") while the anus formed later as an opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the slit-like blastopore close up in the middle, leaving openings at both ends that become the mouth and anus. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「mouth」の詳細全文を読む スポンサード リンク
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