Animals go through significant morphological and physiological change from hatching or birth to adulthood. Despite these transitions, fundamental behaviors such as feeding and locomotion must persist, but their performance may necessitate alternative strategies and structures. Relationships between anatomical structure and behavioral modality have been examined widely across species at the adult life stage, but less attention has been given to how these relationships change through life history. Examining relationships between structure and behavior across life history can provide insight into the evolution of behaviors as well as the functional plasticity that structures may exhibit. In this thesis, I utilize the Mauthner-based startle response in teleost fish and frogs as a model system for examining changes in morphology and behavior through life history. In fish, which have a well-characterized startle, I examine body shape and aspects of neural architecture and relate them back to what we already know about the behavior. In my second chapter I identify an ontogenetic change in body shape that is prevalent across ray-finned fish. In my third chapter, I characterize rostrocaudal regionalization within the larval mechanosensory cell population in zebrafish. Frogs do not have a well-characterized startle repertoire through metamorphosis, so in my fourth chapter I describe how a species of pipid frogs maintains a startle response as it transitions from axial- to limb-based locomotion. In my fifth chapter I discuss the teleost fish and frog systems, and propose experiments that could more directly examine the structure-function relationships that my findings indicate are changing through ontogeny.