Water bears are small microscopic invertebrates that grow to only about 1mm in length. Over 400 species have been identified since 1773.  They are also known as Tardigrades which means "slow stepper" named by Italian scientist Spallanzani in 1776, and classified as phylum Tardigrada. They have eight legs with four claws and are well known as tough animals that can survive in extreme environments (extremophiles).
Structure of Water bears
Water Bears are tiny structures that are bilaterally symmetrical. Water bears have a round head which includes a part of their mouth, the shape of their mouth is sharp and pointy, and behind the mouth opening,they have two calcareous stilettos that can pierce their food and it consists of a sucking tube. They usually have one pair of eye pigment spot on their head. Water bears usually have four pairs of legs, and around four to eight claws attach to each leg. The colors of the water bears vary depending on their species,and consists of the colors gray, bluish, yellowish-brown, reddish, and brown.
The food enters from the anterior mouth that are connected to the salivary glands and stylet apparatus. The stiffening of the mouth by the rings of cuticle allows the food to enter from the mouth which then, goes down the pharynx, esophagus and it goes through midgut where the food is absorbed. When the secretions are added to the midgut, the hindgut finally expels wastes through the anus.
Respiratory System and Circulatory system
Water bears have a special dorsal excretory gland and two Malpighian tubules on their body but they usually depend on the other organs in the body. Simple wastes go through the wall of midgut and expels. Other wastes that are expelled by molting, they usually keep wastes in the cuticle.
Female water bears have a larger body structure than males and are also more numerous. They reproduce both sexually and asexually. Asexual reproduction includes, parthenogenesis, hermaphrodism, which means that they have characteristics and sex organs of both male and female genders and gonochoristic which means the sex is determined by factors other than genetics. . In sexual reproduction, the male’s sperm goes in to female’s body. And they are oviparous.
Water is necessary for water bear to live! Water bears are aquatic for supplying layer water to active the body, water bear can be found in marine, fresh water area and semi-aquatic terrestrial environments. 
Water bears usually eat plant or animal cells. For example, they feed on bacteria, algae, decayed plants. In addition, they get small invertebrates, if some of them are carnivorous. Amoebas, nematodes, and other water bear feed on water bear.
Water bears can survive freezing for a few months or years. For example, they are found from 6000 meters up in Himalaya as well as Antartic regions. In addition, some of them can resist in about absolute zero which the degree is around -272ºC. Moreover, water bears can be adapted to hot condition as around 150°C.
Water Bear Media
Water bear is walking and eating moss
Water bear can survive in extreme environment
- Tardigrada by Wikispecies.
- PHYLUM TARDIGRADA by Wesley R. Elsberry,Online Zoologists ,Spring 1982
- Tardigrades (Water Bears) Sarah Bordenstein, Microbial Life
- Tardigrada by Palaeos
- Tardigrada by Paul D. N. Hebert, Leszek A. Bledzki(editor), Biodiversity Institute of Ontario,August 25, 2008