On the fine structure of the creeping larva of Loxosomella murmanica: additional evidence for a clade of Kamptozoa (Entoprocta) and Mollusca

The creeping larva of the kamptozoan (entoproct) Loxosomella murmanica was investigated using transmission electron microscopy. The late larva exhibits a prominent apical organ connected to the ‘cerebral’ commissure of large cerebral ganglia, which supply the paired frontal organ. From the cerebral ganglia two paired nerve cords project backwards, closely resembling the tetraneuralian pattern of basal molluscs. In addition, a neural ring supplying the prototroch is present. The epidermis is composed of myoepithelial cells. Dorsally its cuticle is covered by granules of unknown composition. The prototroch consists of two ciliary rings; a downstream collecting system is not present. Although there is a one‐way gut with a lumen throughout, the larva obviously does not feed. A single pair of protonephridia is present. The foot sole shares distinct similarities with basic molluscs, particularly with those of the aplacophoran Solenogastres: The anterior part shows a huge, subepidermal pedal gland and several bundles of cirri consisting of compound cilia. The posterior part is ciliated with intraepithelial mucous cells interspersed. The dorsoventral muscle fibres show the mollusc‐like ventral intercrossing. The present results and previous findings, in particular the chitinous, non‐moulted cuticle, the sinus circulatory system, and a number of neural features shared by Kamptozoa and Mollusca, provide substantial evidence for a direct sister‐group relationship between these phyla. In addition, the basal position of the Solenogastres (Neomeniomorpha) within the Mollusca is corroborated.