Mollusks are a highly diverse phylum with some of the most extremes of body forms from wormlike Aplacophora to tentacled cephalopods. The relationship between phenotype and phylogeny is studied here in two very different groups of mollusks, the bivalves and the terrestrial gastropods. The eastern oyster, Crassostrea virginica, being a nearly amorphic organism, expresses its phenotype as physiological responses to its environment. This ancient group has a huge distribution over a range of climates and conditions, lives either submerged or exposed along the edge of the water and faces assault from storms and floods that make its habitat one of the harshest on earth. The physiology of the eastern oyster is investigated to determine its response as a physiological phenotype to different tidal elevations, salinities, and environmental conditions, including the aftermath of a hurricane and a bacterial pathogen. This research investigates the immunity gene expression and mitochondrial genes to determine if selection has any effect in the oyster response. Triodopsis snails, on the other hand, generally have small ranges that appear to be similar habitats but have many morphological forms that may have evolved in sympatry. The morphology, biogeography and phylogenetics of the Triodopsis genus is studied to determine the status of recognized species, and probable evolutionary history. Both groups of mollusks have incredible adaptations to their respective lifestyles. Oysters have an amazing ability to acclimate to a wide range of conditions without changes in their genetics, whereas the snails have mtDNA that appears to function in spite of extreme variation and holds evidence of volatile climate change and migration of a slow animal. The oyster and the snail are extreme examples of diversity and adaptation to adverse habitats and conditions with different life strategies.