Byrne Maria, Nakajima Yoko, Chee Francis C, Burke Robert D
Department of Anatomy and Histology, F13, University of Sydney, NSW 2006, Australia.
Evol Dev. 2007 Sep-Oct;9(5):432-45. doi: 10.1111/j.1525-142X.2007.00189.x.
The anatomy and cellular organization of serotonergic neurons in the echinoderm apical organ exhibits class-specific features in dipleurula-type (auricularia, bipinnaria) and pluteus-type (ophiopluteus, echinopluteus) larvae. The apical organ forms in association with anterior ciliary structures. Apical organs in dipleurula-type larvae are more similar to each other than to those in either of the pluteus forms. In asteroid bipinnaria and holothuroid auricularia the apical organ spans ciliary band sectors that traverse the anterior-most end of the larvae. The asteroid apical organ also has prominent bilateral ganglia that connect with an apical network of neurites. The simple apical organ of the auricularia is similar to that in the hemichordate tornaria larva. Apical organs in pluteus forms differ markedly. The echinopluteus apical organ is a single structure on the oral hood between the larval arms comprised of two groups of cells joined by a commissure and its cell bodies do not reside in the ciliary band. Ophioplutei have a pair of lateral ganglia associated with the ciliary band of larval arms that may be the ophiuroid apical organ. Comparative anatomy of the serotonergic nervous systems in the dipleurula-type larvae of the Ambulacraria (Echinodermata+Hemichordata) suggests that the apical organ of this deuterostome clade originated as a simple bilaterally symmetric nerve plexus spanning ciliary band sectors at the anterior end of the larva. From this structure, the apical organ has been independently modified in association with the evolution of class-specific larval forms.
棘皮动物顶器中5-羟色胺能神经元的解剖结构和细胞组织在双羽幼虫型（耳状幼虫、双羽幼虫）和长腕幼虫型（蛇尾长腕幼虫、海胆长腕幼虫）幼虫中呈现出类特异性特征。顶器与前部纤毛结构相关形成。双羽幼虫型幼虫的顶器彼此之间比与任何一种长腕幼虫型的顶器更相似。在海星双羽幼虫和海参耳状幼虫中，顶器跨越纤毛带区域，这些区域横穿幼虫的最前端。海星的顶器还有突出的双侧神经节，与神经突的顶网络相连。耳状幼虫的简单顶器与半索动物柱头幼虫的顶器相似。长腕幼虫型的顶器明显不同。海胆长腕幼虫的顶器是位于幼虫臂之间口笠上的单一结构，由两组通过连合连接的细胞组成，其细胞体不在纤毛带中。蛇尾长腕幼虫有一对与幼虫臂的纤毛带相关的外侧神经节，可能是蛇尾类的顶器。有袋类（棘皮动物+半索动物）双羽幼虫型幼虫中5-羟色胺能神经系统的比较解剖学表明，这个后口动物类群的顶器起源于一个简单的双侧对称神经丛，跨越幼虫前端的纤毛带区域。从这个结构开始，顶器随着类特异性幼虫形态的进化而独立地发生了改变。
