The Cowrie is an aquatic snail whose shell is prized among shell collectors but whose biology is mostly not understood. Here we present a novel explication of both body and shell ontogeny emergent from mechanical constraints and a behavioral change. When sexually mature the snail extrudes its mantle through the shell aperture effectively turning its exoskeleton into an endoskeleton. This behavioral shift, in turn, changes the form of both the body and the shell. We describe quantitatively three novel features of the cowrie shell: deviation of its central spiral, labial thickening around the aperture, and ridges forming along said aperture (teeth). We couple elasticity, growth, and shell deposition in biophysical models to predict regimes of mantle mechanical properties consistent with the reported geometrical data. These models integrate and replicate the aforementioned shell features while enabling the charting of a morphospace suggesting why some evolutionarily related species, like the Ovulidae, have similar shell features while lacking apertural teeth. Our recurrent morphological models can be tied into the known evolutionary divergence of the family to select locations of probable discontinuities of mantle biophysical properties. Beyond this reduced modeling we also solve and show three dimensional renderings of cowrie shells based on our novel mechanical framework. Our work suggests a closed mechanical and behavioral feedback loop steering the development of a juvenile cowrie to its adult form.