New data on echiuran anatomy and histology: the case of Lissomyema mellita (Annelida: Thalassematidae).

Echiura is small group of unsegmented marine worms that are sometimes abundant in the benthos of all areas of the World Ocean. The study of echiuran morphology and anatomy is important for understanding echiuran biology and the function of benthic communities where echiurans dominate. The current study used paraffin histology, scanning electron microscopy, and 3D reconstruction to describe the anatomy of organ systems in Lissomyema mellita, which is within the tribe Thalassematini of the subfamily Thalassematinae. The body of L. mellita consists of a trunk and a proboscis. The trunk terminates at the anal lobe, which contains many large glands, the secretions of which may be used for processing of the hard substratum where studied animals were found. The proboscis has lateral ciliated grooves that are < 7 μm wide and are probably used for collecting and separating food particles. The coelom is divided into the coelom of the proboscis and the coelom of trunk. The location of proboscis coelom may suggest that the proboscis is derived from the oral segment of the body of a hypothetical segmented echiuran ancestor. The shortness and histological uniformity of the foregut indicates that L. mellita feeds on soft food particles that do not require mechanical processing. The circulatory system has a complex organization and contains several vascular elements that have not been previously described in echiurans: voluminous plexus around the foregut, voluminous dorsal and ventral lacunes, and additional vessels between neurointestinal and circular blood vessels. The dorsal blood vessel is very short; the circumchaetael vessel is absent. Most of the anatomical peculiarities of L. mellita appear to be related to its biology. The newness of the data on the organization of the circulatory system reflects the general scarcity of knowledge on echiuran anatomy. The newly discovered features should be incorporated into general schemes of echiuran organization.