Pulmonates

Pulmonates (Pulmonata) are an informal group of gastropods that include air-breathing snails and slugs. Most pulmonates are terrestrial or freshwater species, although there are a few marine species as well.

Pulmonates differ from other molluscs in their method of breathing. Over the course of their evolution, their gills have been replaced by a lung that has developed in the mantle cavity. The mantle cavity wall, which forms the lung sack (also called a pallial lung), has become highly vascularized (full of blood vessels). The mantle muscles expand and contract to enable air to flow in and out of the lung through a single opening on the right side of the mantle cavity called the pneumostome (or breathing pore). Oxygen is absored through the lung tissues and carbon dioxide is exhaled.

Many pulmonates have a coiled shell (although some groups such as the slugs have lost their shell). Pulmonates have one or two pairs of tentacles on their head (generally, terrestrial species have two pairs and aquatic species have one pair). Olfactory organs are located on the tips of the tentacles.

Pulmonates that live on dry land such as terrestrial snails and slugs face the challenge of conserving water and avoiding dessication. As a result, they have undergone adaptations to minimize water loss. For example, their excretory systems convert nitrogenous wastes to uric acid that is secreted as solid crystals instead of urine (which contains water).

Pulmonates are hermaphroditic (each individual has both male and female reproductive organs). Beyond that, their reproductive anatomy and behavior vary widely. An unusual characteristic of the reproductive habits of terrestrial snails and slugs is the use of love darts. A love dart (also called a gypsobelum) is a long, calcarous (or chitnious) dart-like structure. Prior to copulation, the snails (or slugs) try to embed their love darts into their mate. The love dart does not transfer sperm but serves only as part of courtship behavior. The transfer of sperm between the snails occurs later in the mating ritual.

The classification of gastropods, and many mulluscs for that matter, remains unclear and is the subject of ongoing debate and study. Relationships between the various groups of gastropods are frequently reinterpreted as new data emerges. For this reason, groups once thought to be closely related are found to be more distant relatives and vice versa. In some cases groups once useful in describing the species they included are abandonned or replaced in light of new information. This is the case with the pulmonates.

Although the pulmonates were once considered to be a taxonomic class, the group is no longer thought to reflect a single lineage. For this reason, the term "pulmonate" is now used informally only. The newer taxonomic class, Heterobranchia, now includes many of the species that the former Pulmonata clade included.

Despite the fact that pulmonates are no longer considered to be a true taxonomic class, we can still consider the various groups that are generally referred to as pulmonates. These include three groups, the Stylomatophora, the Basommatophora and the Archaeopulmonata.

The Stylommatophora are land pulmonates and include more than 15,000 species that inhabit a range of drier terrestrial habitats such as deserts, grasslands and forests. The Basommatophora are freshwater pulmonates that inhabit a spectrum of wet habitats—freshwater streams, lakes and wetlands. There are also some species of Basommatophora that inhabit marine intertidal zones. The Archaeopulmonata are a primitive group of gastrpods that have coiled shells. The species Otina ovata belongs to this group—it is an air-breathing sea snail that inhabits shorelines throughout the British Isles and western Europe.

Gastropod evolution is still much debated but it is generally thought that gastropods first appeared during the Cambrian. Pulmonates are thought to have evolved during the Carboniferous Period.