Terrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water (e.g., fish, lobsters, octopuses), or amphibians, which rely on a combination of aquatic and terrestrial habitats (e.g., frogs). Terrestrial animals evolved from marine animals (aquatic animals living in the ocean). The term terrestrial is also frequently used for species that live primarily on the ground, in contrast to arboreal species, which live primarily in trees.
Terrestrial animals do not form a unified clade, rather they share only the fact that they live on land. The transition from an aquatic to terrestrial life has evolved independently and successfully many times by various groups of animals.
When excluding internal parasites, free living species in terrestrial environments are represented by the following ten phyla; Flatworms (Planaria), Nemertea (ribbon worms), Nematoda (roundworms), Rotifers, Tardigrada (water bears), Onychophora (velvet worms), Arthropods, mollusks (gastropods), Annelida and Chordata (tetrapods). The roundworms, tardigrades and rotifers are microscopic animals that requires a film of water to live in, and are not considered truly terrestrial. Flatworms, ribbon worms, velvet worms and annelids all depends on more or less moist habitats, while the three remaining categories, arthropods, gastropods and tetrapods, are the only ones that contain species that have been able to adapt to predominantly dry terrestrial environments.
Labeling an animal species "terrestrial" or "aquatic" is often obscure and becomes a matter of judgment.
Many animals which are considered terrestrial have a life-cycle that is partly dependent on being in water. Penguins, seals and walruses sleep on land and feed in the ocean, yet they are all considered terrestrial. Many insects and all terrestrial crabs (as well as other clades) have an aquatic life cycle stage: their eggs need to be laid in and to hatch in water. After hatching there is an early aquatic form, either a nymph or larva.
There are crab species which are completely aquatic, crab species which are amphibious, and crab species which are terrestrial. Fiddler crabs are called “semi-terrestrial” since they make burrows in the muddy substrate to which they retreat during high tides. When the tide is out, fiddler crabs search the beach for food.
The same is true in the Mollusca: many hundreds of gastropod genera and species live in intermediate situations, such as for example, Truncatella. Some gastropods with gills live on land, and others with a lung live in the water.
As well as the purely terrestrial and the purely aquatic animals there are many borderline species. There are no universally accepted criteria for deciding how to label these species.