Although in most of the modeling methodologies the structure of the models is viewed as time-invariant, dynamic structure models offers a better paradigm to represent certain type of systems. We describe the Dynamic Structure Discrete Event System Specification (DSDEVS) formalism and its implementation the DELTA modeling and simulation environment. The DSDEVS formalism is used to model the Eratosthenes Sieve for prime numbers. The sieve is modeled in two different perspectives: iterative and recursive. These approaches are compared with respect to performance and model expressiveness. A variation of the iterative solution will be extended to find relatively prime numbers in a scrambled list, showing the generality of the DELTA system.