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Terrestrial animals areanimals that live predominantly or entirely onland (e.g.,cats,chickens,ants, mostspiders), as compared withaquatic animals (e.g.,fish,whales,octopuses,lobsters, etc.), who live predominantly or entirely inbodies of water; andsemiaquatic animals (e.g.,crocodilians,seals,platypus and mostamphibians), who inhabitcoastal,riparian orwetland areas and rely on both aquatic and terrestrialhabitats. While mostinsects (who constitute over half of all knownspecies in the animalkingdom^[1][2]) are terrestrial, some groups, such asmosquitoes anddragonflies, spend theiregg andlarvalstages in water but emerge as fully terrestrialadults (imagos) after completingmetamorphosis.

Terrestrial animals conductrespiratorygas exchange directly with theatmosphere, typically via specializedrespiratoryorgans known aslungs, or viacutaneous respiration across theskin. They have also evolvedhomeostatic features such as impermeablecuticles that can restrictfluid loss, temperature fluctuations andinfection, and anexcretory system that can filter outnitrogenous waste in the form ofurea oruric acid, in contrast to theammonia-based excretion of aquatic animals. Without thebuoyancy of an aqueous environment to support theirweight, they haveevolved robustskeletons that can hold up their body shape, as well as powerfulappendages known aslegs orlimbs to facilitateterrestrial locomotion, although some performlimbless locomotion using body surface projections such asscales andsetae. Some terrestrial animals even havewings ormembranes that act asairfoils to generatelift, allowing them tofly and/orglide asairborne animals.

In a narrower sense, the word "terrestrial" is used to specifically describe animals that live on the ground (particularly those living obligately on thesoil surface), as opposed toarboreal animals that live intrees, even though trees, like theshrubs andgroundcovers from the lowerlayers, are all an integral component of theterrestrial ecosystem.

The term "terrestrial", in a more specific sense, is typically applied to species that live primarily on the ground or inburrows inside thesoil, in contrast toarboreal species, who live primarily intrees, even though the latter are actually a specialized subgroup of the terrestrialfauna.

There are other less common terms that apply to specific subgroups of terrestrial animals:

• Saxicolous organisms arerock-dwelling. "Saxicolous" is derived from the Latin wordsaxum, meaning a rock.
• Arenicolous organisms live in thesand.
• Troglofauna are organisms that predominantly live incaves.

Terrestrial invasion is one of the most important events in thehistory of life.^[3][4][5] Terrestrial lineages evolved in several animalphyla, among which arthropods, vertebrates and mollusks are representatives of more successful groups of terrestrial animals.

Terrestrial animals do not form a unifiedclade; rather, they are apolyphyletic group that share only the fact that they live on land. The transition from an aquatic to terrestrial life by various groups of animals has occurred independently and successfully many times.^[5] Most terrestrial lineages originated under a mild ortropical climate during thePaleozoic andMesozoic, whereas few animals became fully terrestrial during theCenozoic.

If internalparasites are excluded, eleven phyla include free living species in terrestrial environments. These can be grouped as follows:

Three phyla contain species that haveadapted totally to dry terrestrial environments, and which have no aquatic phase in their life cycles:

• Arthropods — fully terrestrial members:hexapods (crown group beinginsects),arachnids,myriapods, and some land-evolvedcrustaceans such aswoodlice,sandhoppers,coconut crab andterrestrial crabs; semi-terrestrial members include most species ofcrabs,crayfish,water fleas,copepods, andseed shrimp
• Molluscs — mainlyterrestrialgastropods:land snails andslugs; one species ofcoleoidcephalopod, thealgae octopus, is also known to be routinely terrestrial
• Chordates — specificallytetrapodvertebrates (especiallyamniotes); semiterrestrial members: non-caecilianamphibians (frogs andsalamanders) andamphibious fish

Four phyla include species that depend on more or less moist habitats:

• Annelids — mainlyoligochaeteclitellates such asearthworms,pot worms and someleeches, but require moist soil habitats, highly diverse and derived fromtheir marine relatives
• Onychophores (velvet worms) — the only solely terrestrial phylum, though require moist habitats and have restricted range
• Platyhelminthes (flatworms) — specificallyland planarians, require moist habitats and have restricted range
• Nemerteans (ribbon worms) — specifically the 12 terrestrial species from the suborderMonostilifera, require moist habitats and have restricted range

Species in four more phyla, as well as some smaller species of arthropods and annelids, are microscopic animals that require a film of water to live in, and are therefore considered semi-terrestrial:^[6]

• Gastrotrichs (hairy-backs) — live in transient terrestrial water and go dormant during desiccation
• Rotifers (wheel animals) — live in transient terrestrial water and go dormant during desiccation
• Nematodes (roundworms) — mostlyparasitic but some (e.g.Caenorhabditis) are free-livingdetritivores, live intopsoil and go dormant during desiccation
• Tardigrades (water bears) — live in transient terrestrial water and go dormant during desiccation

Labeling an animal species "terrestrial" or "aquatic" is often obscure and becomes a matter of judgment. Many animals considered terrestrial have a life-cycle that is partly dependent on being in water.Penguins,seals, andwalruses sleep on land and feed in the ocean, yet they are all considered terrestrial. Many insects, e.g.mosquitos, and allterrestrial 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 anymph orlarva.

There are crab species that are completely aquatic, crab species that are amphibious, and crab species that 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 themollusca. Many hundreds ofgastropod 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, thus some assignments are disputed.

Fossil evidence has shown that sea creatures, likely arthropods, first began to make forays onto land around 530 million years ago, in the EarlyCambrian. There is little reason to believe, however, that animals first began living permanently on land around that time. A more likely hypothesis is that these early arthropods' motivation for venturing onto dry land was to mate (as modern horseshoe crabs do) or to lay eggs out of the reach of predators.^[7]

Three groups of arthropods had independently adapted to land by the end of the Cambrian:myriapods,hexapods andarachnids.^[8] By the lateOrdovician, they may have fully terrestrialized. There are other groups of arthropods, all frommalacostracan crustaceans, which independently became terrestrial at a later date:woodlice,sandhoppers, andterrestrial crabs. Additionally, the sisterpanarthropodan groupsOnychophora (velvet worms) are also terrestrial, while theEutardigrada are also adapted for land to some degree; both groups probably becoming so during theEarly Devonian.^[9] Among arthropods, many microscopic crustacean groups likecopepods andamphipods andseed shrimp can go dormant when dry and live in transient bodies of water.^{[citation needed]}

By approximately 375 million years ago^[5] the bony fish best adapted to life in shallow coastal/swampy waters (such asTiktaalik roseae). Thanks to relatively strong, muscular limbs (which were likely weight-bearing, thus making them a preferable alternative to traditional fins in extremely shallow water),^[10] and lungs which existed in conjunction with gills,Tiktaalik and animals like it were able to establish a strong foothold on land by the end of the Devonian period. In theCarboniferous, tetrapods (losing their gills) became fully terrestrialized, allowing their expansion into most terrestrial niches, though later on some will return to being aquatic and conquer the air also.

Gastropod mollusks are one of the most successful animals that have diversified in the fully terrestrial habitat.^[11] They have evolved terrestrial taxa in more than nine lineages.^[11] They are commonly referred to asland snails andslugs.

Terrestrial invasion of gastropod mollusks has occurred inNeritopsina,Cyclophoroidea,Littorinoidea,Rissooidea,Ellobioidea,Onchidioidea,Veronicelloidea,Succineoidea, andStylommatophora, and in particular, each of Neritopsina, Rissooidea and Ellobioidea has likely achieved land invasion more than once.^[11]

Most terrestrialization events have occurred during thePaleozoic orMesozoic.^[11] Gastropods are especially unique due to several fully terrestrial and epifaunal lineages that evolved during theCenozoic.^[11] Some members of rissooidean familiesTruncatellidae,Assimineidae, andPomatiopsidae are considered to have colonized to land during the Cenozoic.^[11] Most truncatellid and assimineid snails amphibiously live inintertidal andsupratidal zones frombrackish water to pelagic areas.^[11] Terrestrial lineages likely evolved from such ancestors.^[11] Therissooidean gastropod familyPomatiopsidae is one of the few groups that have evolved fully terrestrial taxa during the late Cenozoic in theJapanese Archipelago only.^[11] Shifts from aquatic to terrestrial life occurred at least twice within two Japanese endemic lineages in Japanese Pomatiopsidae and it started in theLate Miocene.^[11]

About one-third of gastropod species are terrestrial.^[12] In terrestrial habitats they are subjected to daily and seasonal variation in temperature and water availability.^[12] Their success in colonizing different habitats is due to physiological, behavioral, and morphological adaptations to water availability, as well as ionic and thermal balance.^[12] They are adapted to most of the habitats on Earth.^[12] The shell of a snail is constructed ofcalcium carbonate, but even in acidic soils one can find various species of shell-less slugs.^[12] Land-snails, such asXerocrassa seetzeni andSphincterochila boissieri, also live in deserts, where they must contend with heat and aridity.^[12] Terrestrial gastropods are primarily herbivores and only a few groups are carnivorous.^[13] Carnivorous gastropods usually feed on other gastropod species or on weak individuals of the same species; some feed on insect larvae or earthworms.^[13]

Semi-terrestrial animals are macroscopic animals that rely on very moist environments to thrive, they may be considered a transitional point between true terrestrial animals and aquatic animals. Among vertebrates, amphibians have this characteristic relying on a moist environment and breathing through their moist skin while reproducing in water.

Many other animal groups solely have terrestrial animals that live like this:land planarians,land ribbon worms,roundworms (nematodes), andland annelids (clitellates) who are very primitive andbreathe through skin.

Clitellates or terrestrial annelids demonstrate many unique terrestrial adaptations especially in their methods of reproduction, they tend towards being simpler than their marine relatives, thebristleworms, lacking many of the complex appendages the latter have.

Velvet worms are prone to desiccation not due to breathing through their skin but due to their spiracles being inefficient at protecting from desiccation, like clitellates they demonstrate extensive terrestrial adaptations and differences from their marine relatives including live birth.

Many animals live in terrestrial environments by thriving in transient often microscopic bodies of water and moisture, these includerotifers andgastrotrichs which lay resilient eggs capable of surviving years in dry environments, and some of which can go dormant themselves. Nematodes are usually microscopic with this lifestyle. Although eutardigrades only have lifespans of a few months, they famously can enter suspended animation during dry or hostile conditions and survive for decades, which allows them to be ubiquitous in terrestrial environments despite needing water to grow and reproduce. Many microscopic crustacean groups likecopepods andamphipods andseed shrimps are known to go dormant when dry and live in transient bodies of water too.^[6]

• Clack J. A. (2002).Gaining ground: the origin and evolution of tetrapods.Indiana University Press, 369 pp.,ISBN 978-0-253-34054-2.
• Cloudsley-Thompson J. L. (1988).Evolution and adaptation of terrestrial arthropods.Springer, 141 pp.,ISBN 978-3-540-18188-0.
• Dejours P. et al. (1987).Comparative physiology: life in water and on land. Liviana Editrice, Italy, 556 pp.,ISBN 978-0-387-96515-4.
• Gordon M. S. & Olson E. C. (1995).Invasions of the land: the transitions of organisms from aquatic to terrestrial life.Columbia University Press, 312 pp.,ISBN 978-0-231-06876-5.
• Little C. (1983).The colonisation of land: Origins and adaptations of terrestrial animals.Cambridge University Press, Cambridge. 290 pp.,ISBN 978-0-521-25218-8.
• Little C. (1990).The terrestrial invasion. An ecophysiological approach to the origin of land animals. Cambridge University Press, Cambridge. 304 pp.ISBN 978-0-521-33669-7.
• Zimmer, Carl (1999).At the Water's Edge : Fish with Fingers, Whales with Legs, and How Life Came Ashore but Then Went Back to Sea. New York: Touchstone.ISBN 0684856239.

This article incorporates CC-BY-2.0 text from the reference^[11] and CC-BY-2.5 text from the reference^[12] and CC-BY-3.0 text from the reference^[13]

• Zoology
• Animals by adaptation

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