Document Type

Honors Project

Publication Date



The process of embryonic neurogenesis in pond snails can be separated in to pre- and post-metamorphic stages based on the morphology, function, and persistence of neural structures. This distinction is most likely a byproduct of the evolution of direct-development, in which larval stages are undergone entirely inside of the egg capsule. Larval neural structures that were present in the free-living larvae of ancestral species have been adapted to meet the needs of extant encapsulated larvae. Remarkably, the resultant vestigial larval nervous system is functional and physically present alongside the embryonic neural elements that go on to form the adult nervous system.

This study compares embryonic development of the nervous system in the Planorbid and Lymnaeid families of pond snails by reviewing work done on the Planorbid Helisoma trivolvis and the Lymnaeid Lymnaea stagnalis and providing new information about the Planorbid Biomphalaria glabrata. A description of 5-hydroxytryptamine and FMRFamide patterning during embryonic development is provided by immunofluorescence. Apparent overlap of peptidergic and aminergic neural elements after metamorphosis suggests that gangliogenesis may begin embryonically in B. glabrata. Functional studies complement the localization of 5-HT to reveal an apparent vestigial larval nervous system in B. glabrata. These findings suggest a pattern of divergence in the evolutionary history of pond snails that correlates with observable features in the larval nervous systems of extant species.

Level of Honors

magna cum laude




Judith Humphries