Also called Water Bear or Moss Piglet, any of about 350 species of free-living invertebrates belonging to the phylum Tardigrada: their very own. (The scientific name refers to their sluggish gait.)

Tardigrades are mostly about 1 mm or less in size. They live in varying habitats: in damp moss, on flowering plants, in sand, in freshwater, and in the sea. They are especially prevalent in temporary puddles, the small amounts of rain water that collect in cement urns in cemetaries and similar sites. This is not because they chose these habitats - they move passively from place to place, blown on the wind - but because there is so little competition there. Presumably tardigrades also are introduced into permanent ponds, but the numerous and efficient predators in such environments make survival there problematic. In adapting to a wide range of external conditions, a large number of genera and species of tardigrades have evolved.

Tardigrades have a well-developed head region and a short body composed of four fused segments, with each segment bearing a pair of short, stout, unjointed limbs generally terminated by several sharp claws. On the head are two eyespots. The animals have no known specialized organs of circulation or respiration, and the alimentary canal traverses the body from end to end. Most plant-eating tardigrades feed by piercing individual plant cells with their stylets (spearlike structures near the mouth) and then sucking out the cell contents. A few tardigrades are predacious carnivores.

The sex life of tardigrades is of special interest. Though many species reproduce sexually, with females and males laying and fertilizing eggs, respectively, in some species there are no males, and parthenogenic reproduction presumably occurs. The young hatch from eggs; except for size, they are exactly like the adults.

Of most interest is tardigrades' ability, when environmental conditions decline (such as, when their tiny puddle dries up) to enter into a state of cryptobiosis in which all activity ceases and metabolism drops to undetectible levels. In this state the animal can withstand temperatures from less than one degree above absolute zero to 150 degrees centigrade; almost total vacuum; immersion in alcohol and other toxic chemicals; and irradiation with large doses of X rays. Tardigrades may resume normal life after up to 5 or more years of cryptobiosis, and it has been estimated that repeated cryptobiotic periods can extend a life span of only a year or so to 60 or 70 years.

They are darling, appealing little things, like tiny bears crawling along on stumpy legs; some people feel a great deal of enthusiasm for them. Interested persons should visit the self-styled Tardigrade Appreciation Center where there are numerous references, and even information on how to farm them.