Study on plesiomorph and phylogeny of "living fossil" Ostracoda (Crustacea)

摘要报道了Keijcyoidea属一新种，具有简单的节结、双节触角、透明的体节等多形态特征，命名为Keijcyodea infralittoralis Tsukagoshi， Okada和Horne， 2006。并对其生态性质（生境和行为）进行了分析。此外，在鸭嘴兽类群中首次完整地描述了所有个体发生的附属物和甲壳。在较年轻的幼鱼身上观察到成人缺乏的第七肢。因此，第七肢的缺失是一种自形性特征，而在较年轻的星上的存在表明，鸭嘴兽和足类具有共同的祖先。虽然附属物附加模式与足足类存在较大差异，但这种差异主要来自于粗砂沉积物中的栖息地和行为，而非系统发育的原因。众所周知，普通介形虫使用下颌骨的尖点来打开两个瓣。而扁甲类甲壳类动物，包括下角甲壳类，下颌尖不发育。经过详细的观察，考虑到两个新的事实，即：1)内甲壳表面的分叉点，对应于顶尖点，向前倾斜；2)侧翼前筛分的分叉点与侧翼基部的柄状突相对应。这些事实表明，圆锥鸟不是用下颌骨而是用天线来打开瓣膜。肌肉网络系统的观察也支持了这种差异。阀门开启的方法必须来自饮食习惯的演变。

A new species of genus Keijcyoidea with some plesiomorphic characters such as simple hingement, biramous antenna, clear body segments of urosom, was described as Keijcyodea infralittoralis Tsukagoshi, Okada and Horne, 2006. And its ecological nature (habitat and behavior) was also showed. Furthermore, all appendages and carapaces of all instars through the ontogeny were completely illustrated and described in the first time in Platycopida. The seventh limb which is lacking in adult was observed in younger juvenile. Therefore the lacking of the seventh limb is an autapomorphic character and the existence in younger instar suggests that Platycopida and Podocopida share a common ancestor. Though the appendage additional pattern is largely different from the Podocopida, it is suggested that the difference comes from the habitat and behavior in coarse sand sediments rather than the phylogenetical reason.It is well known that common ostracods use the apical point of mandible for opening the both valves. But the platycopan ostracods including Keijcyodea infralittoralis do not so develop the point of mandible. After detail observations, the two new facts were regarded, i.e. : 1)the furcral point of inner carapace surface, which corresponds to the apical point, sifts to anterior side in platycopans ; 2)the anterior sifted furcral point corresponds to the humb-like process in the basal part of antenna in Platycopida. These facts mean that the platycopans do not use the mandible but the antenna for valve opening. The difference is also supported by the observation of muscle network system. The method for valve opening must come from the evolution of eating habit.