鱼嘴

鱼嘴（fish mouth）也称鱼颚或鱼颌（fish jaw），是构成鱼类的口腔的骨骼、软骨、肌肉和韧带等软组织的组合结构，是鱼类消化道和呼吸道的开端。绝大多数硬骨鱼有两套颌骨——主要的口颌（oral jaws）负责开合嘴部并吞咬食物，而位于咽部后方的咽颌（pharyngeal jaws）则负责咀嚼和吞咽食物。软骨鱼类（比如鲨鱼和鳐鱼）只有由软骨组成的口颌。通常情况下，鱼类的颌骨由关节连接、垂直相对的上颌和下颌组成，并且可以拥有规则排列的牙齿。软骨鱼的口颌则会长出数套多次从内向外移动更换的牙齿。 鱼类的颌骨（特别是硬骨鱼）演化出了类似连杆机构的复杂结构，来适应水域生态系统中的各种竞争需求。比较明显的是能够迅速协调的将颌骨向前突出张开的平面四杆机构，使得鱼嘴可以迅速扩大口腔体积并产生负压将猎物吸入口中。鱼类的前上颌骨也配有这种结构，使得整个鱼嘴拥有三套四杆机构可以前后、上下、左右的口腔直径都产生扩大。 鱼嘴的颌骨结构可能来源于支撑无颌鱼类鳃部的咽弓（pharyngeal arches）。最早的鱼颌出现在约4亿3000万年前的志留纪的（已灭绝的）盾皮鱼和棘鱼之中。拥有可积极开合的颚部最初的选择优势可能并不与进食有关，而是增加呼吸效率——颚部开合产生的“颊泵”（buccal pump）效应可以让更多的新鲜水在单位时间内流过鱼鳃以便增加纳氧量。用开合颌骨进行吞咬很可能只是一个连带产生的副功能，但随后变成了许多从早期鱼类演化出的脊椎动物的主要生存技能，被一些演化生物学家誉为“脊椎动物历史上最深刻并最激进的演化步伐”和“至关重要的创新”。相比之下，无颌鱼类的生存难度更高，因此大部分都在三叠纪灭绝没能存活至今，对少数存活至今的圆口纲鱼类（盲鳗和七鳃鳗）的研究也没能帮助解释早期颌骨的演化对脊椎动物头骨深层重塑的影响。 通常的看法是脊椎动物的颌骨与鱼类的鳃弓（branchial arch或gill arches）同源，都来自于胚胎阶段发展出的咽弓。无颌鱼的鳃裂开口在嘴后方，由软骨组织支撑，而第一组鳃弓则环绕嘴口。这第一组鳃弓在有颌鱼中发生对折弯曲变成了上下颌；第二鳃弓的上部在则变成了负责将颌骨和颅骨连接的舌颌骨（hyomandibula），在真骨鱼中还负责悬挂鳃盖（operculum）。现在被普遍接受的看法是有颌鱼类的祖先是身上有骨质的甲板覆盖、无颌的甲胄鱼。 有颌下门 Fraser, G. 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