Toward the Origin and Evolution of Unique Echiurid Excretory Organs: New Data From Females of Bonellia viridis

Echiurids, as nonsegmented annelids, have an excretory system of a special organization. The excretory system of the echiuran worms is known to consist of ultrafiltration zones on blood vessels and anal sacs. Prior to this study, the fine structure of the anal sacs had been described in detail only for Thalassema thalassemum (Thalassematinae). In contrast, the more complex anal sacs of Bonelliinae, which contain additional structural elements such as tubules, remained unexplored. This study describes the anatomy, histology, and ultrastructure of the anal sacs of Bonellia viridis Rolando, 1822 (Bonelliinae) using a set of modern morphological methods: computer microtomography, araldite histology, scanning and transmission electron microscopy. New data suggest functional implications for structural elements of the anal sacs: the conical part and the neck of the funnel, the tubules, and the end sac. The ciliary funnels are responsible for collecting filtrate to their conical parts and can close at their base, thus preventing reverse flow. According to the ultrastructural data, the inner epithelium of the tubules and the end sac modifies the incoming filtrate in two ways. The inner epithelium of the tubules carries out pinocytosis and accumulates electron-dense granules. The inner epithelium of the end sac has a basal labyrinth consisting of basal processes with numerous mitochondria extending deep into the extracellular matrix and indicates active ion transport. Additional zones responsible for ultrafiltration were identified in the outer epithelium of the anal sac-specifically within the tubule and at the base of the funnel. The origin of the echiurid anal sacs as a result of fusion and multiplication of the metanephridia at the posterior growth zone of metameric annelid-like ancestor is suggested.