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Tardigrade anatomy [3] Tardigrades have a short plump body with four pairs of hollow unjointed legs. Most range from 0.1 to 0.5 mm (0.004 to 0.02 in) in length, although the largest species may reach 1.3 mm (0.051 in). The body cavity is a haemocoel, an open circulatory system, filled with a colourless fluid.
The tardigrade's ability to withstand extremely cold temperatures as a tun is a form of cryptobiosis called cryobiosis. Although the high temperature endurance of tardigrades has been significantly less studied, their cryobiotic response to low temperatures has been well-documented.
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Tardigrades have been shown to respond to different temperature changes at different developmental stages. Specifically, the younger the egg, the less likely it is to survive extreme environments. However, not too long after development, tardigrades demonstrate a remarkable ability to withstand these conditions.
The anatomy of the reproductive system is an important defining feature in distinguishing the different groups of tardigrades. Heterotardigrades have gonoducts that open to the outside through a preanal gonopore, rather than opening into the rectum as in the Eutardigrada .
The Malpighian tubule system is a type of excretory and osmoregulatory system found in some insects, myriapods, arachnids and tardigrades. It has also been described in some crustacean species, [ 1 ] and is likely the same organ as the posterior caeca which has been described in crustaceans.
Tardigrades, which are eight-legged micro-animals, are commonly referred to as water bears or moss piglets and are found all over the world in varying extreme habitats. First discovered in 1904 and originally named Hypsibius antarcticus , Acutuncus antarcticus is the most abundant tardigrade species in Antarctica.
Tardigrades can withstand 1,000 times more radiation than other animals, [17] median lethal doses of 5,000 Gy (of gamma rays) and 6,200 Gy (of heavy ions) in hydrated animals (5 to 10 Gy could be fatal to a human). [18] Earlier experiments attributed this to their lowered water content, providing fewer reactants for ionizing radiation. [18]