The Nervous System and Ciliary Band of Muller's Larva

The newly hatched Muller's larva of the polyclad Pseudoceros canadensis is described at the electron microscopical level with attention to the arrangement and innervation of the ciliary band and the organization of the larval nervous system. Distinctive ultrastructural features allow the trochal cells of the band to be distinguished from general epithelium of the body surface and the specialized oral field epithelium. The band comprises a ventral and a dorsal marginal loop that run along the margins of the six projecting lobes of the larva, and a suboral plate that forms a bridge between these behind the mouth. These three components are joined asymmetrically to form a single, but discontinuous, band: on the left the ventral loop and suboral plate are joined, but these fail to connect with the dorsal loop; on the right it is the ventral and dorsal loops that join, but there is no connection between these and the suboral plate. A system of intraepithelial nerves is associated with the ciliary band, the largest nerves being those in the ventrolateral lobes and the intraepithelial commissure that connects these across the oral field. The system is truly peripheral: it lies outside the basement membrane and is separated by it from the central nervous system, which at this stage comprises a brain and four radiating nerve cords. The peripheral and central nervous systems are in direct contact only at two points, located just behind the ventrolateral lobes on either side of the larva, where a few neurites pass through the basement membrane from one system to the other. The neurites of the peripheral system arise mainly from bipolar sensory cells located in the ciliary band. These are concentrated along the ventrolateral lobes, and their projecting cilia face incoming water currents. Observations on larval swimming behaviour do not, however, suggest any obvious function for these cells. The ciliary band is thus organized as a self-contained unit supplying its own innervation. Other primitive invertebrate larvae have ciliary bands that are similar to some extent in their organization and ultrastructure. This, added to what is already known about Muller's larva, supports the idea that it is primitive and is closely related to at least several other larval types, but it is not clear how the overall arrangement of the band in Muller's larva as described here relates to what is seen in other larvae. Several ways in which the pattern might have originated from simpler patterns in hypothetical ancestral forms are, however, discussed.