Pokedex entries aren't to be taken literally, per say, in the games. No, the pokemon looks nothing like Shellder, but in the anime when Shellder bites it to evolve, both of them change form. However in the games when Slowpoke evolves, Shellder isn't needed or seen. So basically in the games, the pokedex isn't exactly "accurate", like the pokedex says Arcanine is a Legendary Pokemon, when its actually not.