Reconstruction of the neuromuscular system of the swimming-type larva of Loxosomella atkinsae (Entoprocta) as inferred by fluorescence labelling and confocal microscopy

Entoprocta is one of the most enigmatic phyla of the Animal Kingdom. The morphology of their larvae has been little investigated, with details on the larval musculature lacking entirely and immunocytochemical data on the larval nervous system available for only 2 species. Here, we provide the first detailed study of the muscular bauplan and the serotonergic nervous system of an entoproct swimming-type larva. The overall muscular architecture of the larva of Loxosomella atkinsae Bobin & Prenant, 1953 is complex and includes several sets of ring, longitudinal, and diagonal muscles. The dorsal region of the larva (episphere) and the apical organ comprise an outer layer of tightly packed ring muscles. Beneath this layer lie sets of prominent longitudinal and diagonal muscles that run in dorso-ventral direction. The prototrochal musculature is composed of compact layers of outer ring and inner longitudinal muscles. The serotonergic nervous system consists of 3–4 flask-shaped serotonergic cells in the apical organ and a paired nerve passing the frontal neuropil and connecting to the serotonergic prototroch nerve ring. We show here that the entoproct larval stage, in addition to the adult stage, provides several morphological characters for evolutionary inferences. Comparative data on entoproct swimming-type larvae suggest ring and longitudinal muscles underlying the prototroch, a paired main longitudinal muscle, and an unpaired abfrontal longitudinal muscle as part of the ancestral muscular groundpattern of loxosomatid swimming-type larvae.