The term ramshorn snail or ram's horn snail is used in two different ways. In the aquarium trade it is used to describe various kinds of freshwater snails whose shells are planispiral, meaning that the shell is a flat coil. Such shells resemble a coil of rope, or (as the name suggests) a ram's horn. In a more general natural history context, the term "ramshorn snails" is used more precisely to mean those aquatic pulmonate gastropod mollusks in the family Planorbidae that have planispiral coiled shells. Ramshorn snails have been bred for the aquarium trade and various color forms have been selected. The two species commonly found in aquariums are Planorbella duryi and Planorbarius corneus, both in the family Planorbidae. One species (Columbia ramshorn, Marisa cornuarietis) is from a totally different family, the Ampullariidae. Ramshorn snails can sometimes become a nuisance in an aquarium, because some of them breed so profusely. Description Most of these snails are of the family Planorbidae, and they include the species Planorbarius corneus. There are two different coloured skin forms: black and red; the latter lack the dark skin pigment melanin and consequently have a bright reddish skin, which is the colour of their blood. Their blood contains red hemoglobin, unlike other snails' blood, which contains greenish hemocyanin. These ramshorn snails breathe air. Although most of them are extremely small, some may reach a size of two and a half centimeters (one inch). The shells range from translucent through various shades of brown to a dark, nearly black color. The dark color appears to originate from dietary materials not generally available in the home aquarium, although many varieties from ponds are this dark shade. There are two common skin colourations: black and red. The red colouration is the colour of their blood (containing hemoglobin), unlike other snails with green blood (due to hemocyanin). An absence of melanin reveals the red colour. Rare varieties have been selectively bred in pale pink and blue. These traits are recessive and if crossbred the resultant offspring are likely to be wild type (brown). Snails of this family are spiralled sinistrally, with the opening hole slanted downward toward the right. Large folds of skin may protrude out of the more open left side. Like all air-breathing water snails, the animal has no operculum, and has only one pair of tentacles with the eye spots at the base of the tentacles.