The mud snail is black, with a very strong shell, and is about 1/2 inch in length. It is an effective scavenger and hundreds of snails will collect around a dead fish or shrimp bait until they completely cover it. This snail is the host of a larval worm whose adult is a parasite in the blood of wading birds, such as the snowy egret. Normally, the larval worms leave the snails and enter the birds as they wade through the shallows, but the worms also penetrate the skin of humans. Because they are in the wrong host, the larvae quickly die, but because they penetrate the skin, they cause an allergic reaction. Swimmer's itch is a reaction to these larval worms. It usually occurs in the warm summer months near areas where the mud snails are concentrated.