PAGE, L.R.: The Apical Sensory Organ of Gastropod Larvae: New Data on a Patellogastropod

The larval nervous systems of annelid and molluscan larvae include an apical cluster of sensory and non-sensory neurons that is often associated with a tuft of long cilia. Some of the neurons show immunoreactivity to serotonin and the structure includes a neuropil adjacent to the cerebral commissure. In gastropods, this apical sensory organ (ASO, also called apical ganlgion) may control activities of muscles and cilia of the velum and also may be involved in initiating or coordinating metamorphosis. Detailed comparative analyses of this structure may yield characters having value for phylogenetic reconstructions and may provide insights about its function. Among gastropods, the ASO of a number of caenogastropods and heterobranchs have now been described, but the ASO of more basal gastropod clades has not been investigated. I describe the structure of the ASO in several larval and post-larval stages of a patellogastropod, a group that may be the most basal clade of gastropods having living descendants. My observations are based on scanning and transmission electron microscopy and on immunohistochemical techniques for detecting serotonin antigenicity. The ciliary tuft associated with the ASO of Tectura scutum is lost soon after ontogenetic torsion and the neuronal cluster includes at least one and possibly three neurons showing immunoreactivity to serotonin. A serotonergic axon emerges from each side of the ASO and extends along the base of the prototrochal ciliary cells of the velum. These characteristics and additional ultrastructural features of the ASO of T. scutum will be compared to the ASO of the caenogastropod and heterobranch larvae that have been studied to date.