Each dome has six pre drilled and laser sealed holes. These holes are safe and smooth to touch. Although I love my Apple snails, they tend to be a little ravenous. Known to be tank cleaners, they can be very useful at times but not when keeping delicate, oxygen improving live plants, every time I tried with a new plant, they would simply slide right over, rest on the roots and it would be gone in an hour, a tasty snack for the snail but detrimental to my wallet.