2. Examine living pulmonate land snails on display. These snails do not have gills. Instead, the mantle cavity functions like a lung. When a snail is crawling about, what parts of its body are outside the confines of the shell? Would you say that cephalization (evolutionary development of the "head" at the anterior end) is more apparent in gastropods than in bivalves? Could you hypothesize as to why? 3. Move a blunt probe towards the snail you are observing. Which part of the body retracts into the shell first? Notice the disk-shaped plate (operculum) covering the opening of the shell. Does the operculum completely seal of the snail's body from its surroundings when the animal is disturbed? Is the operculum continuous with the shell itself? 4. Snails and slugs secrete a mucus that is laid down by the foot. The slime is protective-a slug can pass unharmed over the edge of a sharp razor - and also contains hormones (pheromones) that guide other snails along the same trail. Can you find the slime trail that is formed as the animal crawls across the substrate?.