Nemertea is a phylum of worms with a simple internal morphology; nemerteans’ spermatozoon morphology can be used for their classification and phylogenetic analyses. The aim of the present study was to describe spermatozoa of the nemerteans Hubrechtella juliae and Sonnenemertes cantelli from the basal groups of the class Pilidiophora at the ultrastructure level. Both species have primitive (‘compact-head’ sensu Stricker and Folsom, 1998) spermatozoa with ovoid head and five mitochondria in the midpiece, but differ in the structure of acrosomal complex: in Hubrechtella juliae, the single lens-shaped acrosomal vesicle contains an area of moderate electron density not enclosed by a separate membrane; in Sonnenemertes cantelli, the acrosome shows a unique morphology and contains a few electron-dense vesicles with irregular shapes and positions and one more electron-lucent elongated vesicle. Such a pattern of the acrosomal complex organization is described for Nemertea for the first time. An assumption is made that the states “two or more mitochondria” and “posterior acrosomal ring component” may be synapomorphies of Hubrechtiiformes+Heteronemertea (class Pilidiophora), whereas “the posterior margin of the acrosomes forms an acrosomal ring component” is presumably an autapomorphy of the family Lineidae s.l. The results suggest that spermatozoa provide a useful source of characters for nemertean systematics.

Nemertea是一种蠕虫门，具有简单的内部形态。nemerteans的精子形态可用于其分类和系统发育分析。本研究的目的是在超微结构水平上描述Pilidiophora类基础群体中的夜蛾Huberchtella juliae和Sonnenemertes cantelli的精子。两种物种均具有原始的精子（'紧凑头'sensu Stricker和Folsom，1998），卵形的卵形头，中段有五个线粒体，但顶体复合物的结构不同：在朱利叶氏菌中，单晶状的顶体囊泡包含一个电子密度中等的区域，没有被单独的膜包围；在Sonnenemertes cantelli，顶体显示出独特的形态，并包含一些具有不规则形状和位置的电子致密囊泡，以及另一个电子透明的细长囊泡。Nemertea首次描述了这种顶体复杂组织的模式。假设状态“两个或多个线粒体”和“后顶体环成分”可能是Hubrechtiiformes + Heteronemertea（类睫毛虫属）的同形，而“顶体的后缘形成了顶体环成分”大概是一种亚同形结果表明，精子为nemertean系统动物提供了有用的特征来源。