Terrestrial animals are animals that live mostly or completely on land, such as cats, chickens, ants, and most spiders. This is different from aquatic animals, like fish, whales, octopuses, and lobsters, which live mostly or completely in water. Semiaquatic animals, such as crocodiles, seals, platypuses, and most amphibians, live in areas near water, like coasts, rivers, or wetlands, and depend on both water and land for survival. While most insects, which make up more than half of all known animal species, live on land, some, like mosquitoes and dragonflies, lay eggs and spend their early life stages in water. These insects later grow into adult forms that live on land after completing a process called metamorphosis.

Terrestrial animals take in oxygen from the air through special organs called lungs or through their skin. They have developed features that help them stay healthy, such as tough outer layers that reduce water loss, protect against temperature changes, and prevent infections. They also have systems to remove waste, like urea or uric acid, unlike aquatic animals, which produce waste in the form of ammonia. Because they do not have the support of water to hold them up, terrestrial animals have strong bones and limbs, such as legs, to move on land. Some move without limbs by using body parts like scales or tiny hair-like structures. A few terrestrial animals, like birds and flying squirrels, have wings or membranes that help them fly or glide through the air.

In a more specific sense, the word "terrestrial" can describe animals that live only on the ground, such as those that stay on the soil surface. This is different from arboreal animals, which live in trees. Even though trees, shrubs, and ground plants are part of the land ecosystem, the term "terrestrial" in this narrower use focuses on ground-dwelling animals.

Ecological subgroups

The word "terrestrial" refers to animals that mainly live on the ground or in underground burrows, compared to arboreal animals, which live mainly in trees. Even though arboreal animals are a specific group within the larger group of terrestrial animals.

There are other, less common terms used to describe specific groups of terrestrial animals.

Taxonomy

One of the most important events in the history of life was the move of animals from water to land. Terrestrial animals evolved in several animal groups, including arthropods, vertebrates, and mollusks, which are examples of successful land-dwelling species.

Terrestrial animals do not all come from the same group; instead, they are a polyphyletic group that only share the trait of living on land. Many different animal groups independently and successfully transitioned from aquatic to terrestrial life. Most of these groups began living on land during the Paleozoic and Mesozoic eras, when climates were mild or tropical. Fewer animals became fully terrestrial during the Cenozoic era.

If internal parasites are not included, eleven animal groups have species that live on land. These groups can be divided into three categories:

• Three groups contain species that live entirely on dry land and do not have any stage in their life cycle that requires water.
• Four groups include species that rely on moist environments.
• Four other groups, along with some small arthropods and annelids, are microscopic animals that need a thin layer of water to survive. These are called semi-terrestrial.

Labeling an animal as "terrestrial" or "aquatic" can be unclear and depends on judgment. Many animals considered terrestrial have life cycles that partly depend on water. For example, penguins, seals, and walruses live on land but feed in the ocean. Insects like mosquitoes and terrestrial crabs also have life stages that require water, such as eggs that must be laid and hatch in water, followed by an aquatic larval or nymph stage.

Some crab species are fully aquatic, some are amphibious, and others are terrestrial. Fiddler crabs are called "semi-terrestrial" because they dig burrows in muddy areas to escape high tides. When the tide recedes, they leave their burrows to search for food on the beach. Similar patterns exist in mollusks. Hundreds of gastropod species live in environments between fully aquatic and fully terrestrial, such as the genus Truncatella. Some gastropods with gills live on land, while others with lungs live in water.

In addition to purely terrestrial or aquatic animals, many species exist in between these categories. There are no clear rules for classifying these species, so some groupings are debated.

Terrestrial panarthropods

Fossil evidence shows that sea creatures, probably arthropods, first began to move onto land around 530 million years ago during the Early Cambrian period. However, there is little evidence to suggest that animals started living permanently on land at that time. A more likely explanation is that these early arthropods went onto dry land to mate, like today's horseshoe crabs, or to lay eggs where predators could not reach them.

By the end of the Cambrian period, three groups of arthropods—myriapods, hexapods, and arachnids—had independently adapted to life on land. By the late Ordovician period, these groups may have lived entirely on land. Other groups of arthropods, all from a type of crustacean called malacostracans, later became terrestrial. These include woodlice, sandhoppers, and terrestrial crabs. Additionally, two groups closely related to arthropods, Onychophora (velvet worms) and Eutardigrada, are also terrestrial or partially adapted to land. These groups likely became land-dwelling during the Early Devonian period. Among arthropods, many tiny crustaceans, such as copepods, amphipods, and seed shrimp, can survive without water for a time and live in temporary water sources.

Vertebrate terrestrialization

Around 375 million years ago, bony fish most suited for life in shallow coastal or swampy waters, such as Tiktaalik roseae, developed traits that helped them survive on land. These fish had strong, muscular limbs that likely supported their weight, making them better than fins for moving in very shallow water. They also had lungs that worked together with gills, allowing them to breathe in both water and air. By the end of the Devonian period, Tiktaalik and similar animals were able to begin living on land. During the Carboniferous period, tetrapods lost their gills and fully adapted to life on land, spreading into many different environments. Later, some of these animals returned to water, showing that life could move between land and water multiple times.

Terrestrial gastropods

Gastropod mollusks are among the most successful animals that have adapted to living on land. These animals have developed many different groups, or lineages, that live fully on land. They are often called land snails and slugs.

Gastropods have moved from water to land in several groups, including Neritopsina, Cyclophoroidea, Littorinoidea, Rissooidea, Ellobioidea, Onchidioidea, Veronicelloidea, Succineoidea, and Stylommatophora. Some groups, like Neritopsina, Rissooidea, and Ellobioidea, likely moved to land more than once.

Most of these changes from water to land happened during the Paleozoic or Mesozoic eras. Gastropods are special because several groups fully adapted to living on land during the Cenozoic era. Some snails in the rissooidean families Truncatellidae, Assimineidae, and Pomatiopsidae are believed to have moved to land during the Cenozoic. Many snails in Truncatellidae and Assimineidae live near water in areas like intertidal and supratidal zones, which are places where salt and fresh water mix. Terrestrial snails likely evolved from ancestors that lived in these areas. The Pomatiopsidae family is one of the few groups that developed fully land-living snails in Japan during the late Cenozoic. In Japan, two groups within Pomatiopsidae shifted from water to land life at least twice, beginning in the Late Miocene.

About one-third of all gastropod species live on land. On land, these animals face changes in temperature and water availability every day and season. Their ability to live in many different places is due to changes in their body functions, behaviors, and body structures that help them manage water, balance ions, and control body heat. These animals can live in nearly all Earth’s habitats. Snail shells are made of calcium carbonate, but some slugs have no shells and live in acidic soils. Some land snails, like Xerocrassa seetzeni and Sphincterochila boissieri, live in deserts and must survive extreme heat and dryness. Most terrestrial gastropods eat plants, but a few eat other animals. Carnivorous gastropods often eat other snails or weak members of their own group, and some eat insect larvae or earthworms.

Semi-terrestrial animals

Semi-terrestrial animals are visible to the naked eye and need very wet environments to survive. These animals are a middle stage between true land animals and fully aquatic animals. Among vertebrates, amphibians share this trait. They live in moist areas, breathe through their wet skin, and lay eggs in water.

Other animal groups include land planarians, land ribbon worms, roundworms (nematodes), and land annelids (clitellates). These animals live on land and breathe through their skin. They are very simple in structure compared to other animals.

Clitellates, or land annelids, have special traits that help them live on land. Their way of reproducing is simpler than that of their ocean relatives, the bristleworms, which have more complex body parts.

Velvet worms are easily affected by drying out. This happens not because they breathe through their skin, but because their spiracles (tiny openings) are not good at keeping them from drying. Like clitellates, velvet worms have many traits that help them live on land. They also give birth to live young, unlike their ocean relatives.

Geoplankton

Many animals live in land environments by surviving in temporary, tiny amounts of water and moisture. These animals include rotifers and gastrotrichs, which lay strong eggs that can survive for many years in dry areas. Some of these animals can also go dormant, or stop being active, when conditions are harsh. Nematodes are usually microscopic and have this same lifestyle. Eutardigrades, which only live for a few months, are famous for entering a state of not being active during dry or dangerous times. This ability allows them to survive for many decades and be found in many land areas, even though they need water to grow and have babies. Small crustaceans, such as copepods, amphipods, and seed shrimps, can also go dormant when dry and live in temporary water sources.