Understanding students' epistemologies of science have become a primary focus for scientific literacy. We want students to be able to reason about evidence and evaluate knowledge claims. This requires an understanding about the epistemology of science and inquiry practices. In this paper, we propose a functional approach for using students' epistemologies to guide inquiry practices. In our design, students use a set of criteria that reflects epistemologies of science and guides construction and evaluation of explanations in their scientific investigations. We use argumentation to create a need for students to use these criteria to compare and evaluate one another's explanations. This study takes place in a 7th grade project-based ecology unit. Our analysis shows that these criteria guide students as they construct and evaluate explanations. During this process, students enhance their epistemologies of science and the quality of their scientific work products.