鳃

腮,大多数水生动物的外部呼吸器官。鱼的鳃位于嘴(咽)后部的鳃腔内。水从嘴中吸进来，被迫通过被称为鳃缝的开口，然后通过鳃裂，即一排精致的鳃之间的空隙，不断地沐浴它们。每一个鳃都是由许多丝状的鳃丝组成，其中含有被薄膜包裹的毛细血管;氧气从经过的水中被吸收，二氧化碳被排出。鳃，可能是板状或簇状，附着在一系列成对的软骨或骨鳃(或鳃)弓的外缘上。鳃耙是鳃弓内缘的骨梳状突出物，它从水中滤出固体物质，防止固体物质通过鳃缝流出，顺着食道流入。除以大型生物为食的鱼类外，所有鱼类都有鳃耙。原始鱼类(如鲨鱼)的鳃缝是露出来的;在硬骨鱼类中，它们由鳃盖或鳃盖保护。 In the higher aquatic invertebrates the gills protrude from the body surface and contain extensions of the vascular system. In the crustaceans these external gills are covered by a protective carapace, part of the shell; in the echinoderms they are branched appendages extending from various parts of the body. In the mollusks the gills (called ctenidia) are internal and are located inside the mantle cavity. Horseshoe crabs have gill books, which are membranous flaps like the pages of a book. Amphibians breathe by means of external gills in their aquatic larval stage; a few forms retain the gills after metamorphosing into terrestrial adults. Aquatic insect larvae accomplish the oxygen–carbon-dioxide exchange by means of tracheal gills, projections from the walls of the air tubes (tracheae); these gills disappear when the insect leaves the water. The embryos of all higher vertebrates pass through a stage in which rudimentary gill slits occur, but these never become functional and disappear as the embryo continues to develop.

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