Phylum Tardigrada (water bears)

tardus= slow gradu= step

1. Bilaterally symmetrical, infinitesimal.
2. Monomeric body, with four pairs of short, unjointed, claw bearing legs upon which the animal crawls.
3. Developed pseudocoelomic body cavity that forms a hydrostatic skeleton.
4. Epidermal layer covered with a permeable cutical that is moulted and may have spines.
5. Two buccal stylets extended through mouth are used to pierce prey.
6. No circulatory system or respiratory system.
7. Nervous system contains a brain and two ventral nerve cords.
8. Cryptobiotic, lose body fluids in conditions that are less than optimal, and go into a tun state.
9. Body divided into a head, three trunk segments, each with a pair of legs, and a caudal segment with the fourth pair of legs.

There are over 400 species of water bears in the phylum Tardigrada. These are grouped into three main classes, Heterotardigrada, Mesotardigrada, and Eutardigrada. Tardigrades can be from 50um to 1200um in length. They are often neglected by zoologists and termed a "minor phyla", but more appropriately should be called a "lesser known phyla". The size, shape, and number of claws are very important in identifying and classifying tardigrades. There are 16 claw types identified. The tardigrades use sexual reproduction, parthenogenesis, and hermaphroditism as modes of reproduction. They are found in marine, freshwater and terrestrial habitats. They can survive almost anywhere using cryptobiosis.

1. First reported by J.A.E. Goeze in 1773 and called "water bears".
2. In a tun state they can survive temperatures as low as -272 up to 150 degrees Celsius.
3. Including suspended animation periods, lifespans can exceed 60 years.

1. Alexander. 1987. Encyclopedia of Animal Biology. Equinox LTD.
2. Barnes, Calow, Olive. 1993. The Invertebrates Blackwell Scientific.
3. Thorp, Couich. 1991. Ecology and Classification of North American Freshwater Invertebrates. Academic Press Inc.