The pharynx connects to a short esophagus, and then to an intestine that occupies much of the length of the body, which is the main site of digestion. The stylets are lost when the animal molts, and a new pair is secreted from a pair of glands that lie on either side of the mouth. The mouth opens into a triradiate, muscular, sucking pharynx. The tubular mouth is armed with stylets, which are used to pierce the plant cells, algae, or small invertebrates on which the tardigrades feed, releasing the body fluids or cell contents. Some tardigrades have three tubular glands associated with the rectum these may be excretory organs similar to the Malpighian tubules of arthropods, although the details remain unclear. No respiratory organs are found, with gas exchange able to occur across the entirety of the body. The body cavity consists of a haemocoel, but the only place where a true coelom can be found is around the gonad. Some species have as many as 40,000 cells in each adult, while others have far fewer. Īll adult tardigrades of the same species have the same number of cells (see eutely). Practically the whole body, except for the last pair of legs, is made up of just the segments that are homologous to the head region in arthropods. In insects, this corresponds to the entire thorax and the abdomen.

Tardigrades lack several Hox genes and a large intermediate region of the body axis. The first three pairs of legs are directed downward along the sides, and are the primary means of locomotion, while the fourth pair is directed backward on the last segment of the trunk and is used primarily for grasping the substrate.

The cuticle contains chitin and protein and is moulted periodically. The legs are without joints, while the feet have four to eight claws each. The body consists of a head, three body segments each with a pair of legs, and a caudal segment with a fourth pair of legs. Most range from 0.3 to 0.5 mm (0.012 to 0.020 in) in length, although the largest species may reach 1.2 mm (0.047 in). Tardigrades have barrel-shaped bodies with four pairs of stubby legs. One tardigrade, Echiniscoides wyethi, may be found on barnacles. Other environments in which they are found include dunes and coasts generally, soil, leaf litter, and marine or freshwater sediments, where they may occur quite frequently, up to 25,000 animals per litre (95,000 animals per gallon). Tardigrades are often found on lichens and mosses, for example by soaking a piece of moss in water. Newly hatched tardigrades may be smaller than 0.05 mm (0.0020 in). The largest adults may reach a body length of 1.5 mm (0.059 in), the smallest below 0.1 mm (0.0039 in). When collected, they may be viewed under a low-power microscope, making them accessible to students and amateur scientists.

Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks. Tardigrades are usually about 0.5 mm (0.020 in) long when fully grown.

The earliest known true members of the group are known from Cretaceous (145 to 66 million years ago) amber, found in North America, but are essentially modern forms, and therefore likely have a significantly earlier origin, as they diverged from their closest relatives in the Cambrian, over 500 million years ago. There are about 1,300 known species in the phylum Tardigrada, a part of the superphylum Ecdysozoa consisting of animals that grow by ecdysis such as arthropods and nematodes. Tardigrades have survived exposure to outer space. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions - such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation - that would quickly kill most other known forms of life. They have been found in diverse regions of Earth's biosphere – mountaintops, the deep sea, tropical rainforests, and the Antarctic. In 1777, the Italian biologist Lazzaro Spallanzani named them Tardigrada ( / t ɑːr ˈ d ɪ ɡ r ə d ə/), which means 'slow steppers'. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär ('little water bear'). Tardigrades ( / ˈ t ɑːr d ɪ ˌ ɡ r eɪ d s/), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals.

Noto philly