The brain and central nervous system of Müller's larva

Abstract

The central nervous system of the Müller's larva of Pseudoceros canadensis is described at the ultrastructural level. It comprises at hatching a brain, apical organ, and four nerve cords. The brain has a central neuropile divided into two distinct regions: an apical plexus of interdigitating neurites and a basal commissure from which the nerve cords arise. The brain closely resembles that of polychaete trochophores in a number of structural and organizational features, particularly in its possession of an apical plexus, evidence for a close evolutionary relationship between Müller's larva and the trochophore. The arrangement of nerve cords suggests a fourfold symmetry may be fundamental to the central nervous system in Müller's larva, but determining whether such symmetry is also reflected in the internal organization of the brain will require further study of early brain development.