Innervation patterns of the cerebral nerves in Haminoea hydatis (Gastropoda: Opisthobranchia): A test for intraspecific variability

This study describes the innervation patterns for the cerebral nerves which project to the cephalic sensory organs (CSOs) in the opisthobranch Haminoea hydatis (Linnaeus 1758) and uses axonal tracing techniques (backfilling) to reveal the central cellular origins for these cerebral nerves. Cell clusters projecting into the cerebral nerves can be defined by their positions in the ganglion relative to other clusters, nerve roots and lobes. The number of cell clusters and the relative sizes of somata are constant in a given cluster, whereas the absolute number of somata and absolute sizes of single somata in a given cluster increase with the size of the animal. Additionally, the invariable morphological characteristics of the cell clusters are used to define criteria for the assessment of possible homology for the clusters innervating the CSOs in Opisthobranchia. The data suggest two different strategies to accommodate the increasing body size; first, the additions of nerve cells and second, the growth of nerve cells.