Etymology:- From the Latin Tardus for slow and Gradu a step, meaning slow stepper

Originally described by Goeze (1773) as "Kleiner Wasser Bar", hence the common name "water bear"

1. Microscopic metazoans, 0.05 to 1.2 mm length.
2. Eight families with characteristics of both Annelida, Arthropoda and many of the minor phyla, including Rotifera, Gastrotricha and Nematoda.
3. Occupy marine, freshwater and moist terrestrial habitats, usually associated with vegetation
4. Bilaterally symmetrical with four pairs of legs with claws at distal ends
5. Limited metameric segmentation, partially coelomate
6. Body covered by a cuticle variously described as chitinous or proteinaceous.
7. Reproductive System:

                Typically sexually reproducing but males unknown in some genera where parthenogenesis occurs

                A single, sac-like gonad occupies the coelom;  other internal organs are suspended in a pseudocoelom

                        In the Eutardigrada, gonoducts open into the rectum

                        In Heterotardigrada, gonoducts open to the outside through a preanal gonopore.

8. Reproduction:

   Two forms of parthenogenesis:.

   1. In diploid tardigrades, meiosis distributes chromosomes between the oocyte and the polar body.
   2. In triploid tardigrades, oocytes are formed by mitotic divisions, yielding triploid eggs.

   Sexual:

   1. Females may lay eggs into the old cuticle after which males ejaculate sperm into the old cuticle.
   2. As a female is molting, males may introduce sperm into the gonopore or cloaca.

9. No  respiratory or circulatory systems.

10. Digestive system:

            a.     Anterior mouth supported by rings of cuticle

            b      Equipped with two stylets for feeding. The stylets are extended to pierce plant or animal cell walls, then the sucking pharynx draws fluids into the digestive system.

            c       Hindgut empties at the posterior anus.

            d.     Feed on fluids inside plant and animal cells, bacteria, algae, fungi, nematodes and small invertebrates

11. Nervous system:

            a. Body weakly cephalized, nervous system has large dorsal cerebral ganglion, sub-esophageal ganglion and paired ventral nerve chords

            b. Brain with lateral lobes connected by two circumpharyngeal cords to a subpharyngeal ganglion.

            c. Ventral ganglia are united by longitudinal nerve chords. Paired nerves from the brain and ganglia innervate the body.

            d. Sense organs:

                    (i)  A pair of red or black pigment eyespots on the head.

                    (ii)  Tactile cirri on head.

12. Excretory System of malpighian tubules in some species
13. Embryology:
    1. Cleavage is total.
    2. Gastrulation occurs by unipolar proliferation.
    3. Five pairs of coelomic pouches form. The posterior pair fuse to form the gonad. The others degenerate and form the buccal apparatus and body muscles.
    4. The embryo undergoes direct development. After hatching, growth occurs by increase in cell size, not number.
    5. Hatching is accomplished by the piercing of the egg by the stylets and legs.
    6. Eutelic with a fixed number of cells in the body of an adult of any given species, usually around 40,000.
14. Ecology and Physiology:
    1. Aquatic and semi-aquatic species in marine, freshwater, and damp terrestrial habitats.
    2. Tardigrades feed on the fluids of plant and animal cells. Some tardigrades are entirely carnivorous.
    3. Tardigrades are prey for amoebae, nematodes, and other tardigrades.
    4. Most species appear to be eurythermal, tolerating temperatures from near freezing to upwards of 30°C.
    5. Food is stored in some epidermal cells.
    6. Respiration occurs by diffusion.
    7. Tardigrades may display cryptobiosis:
       1. Anhydrobiosis occurs under conditions of desiccation. Lose water and take on a wrinkled appearance.
       2. Cysts may be formed when the animal under stress or extreme environmental conditions. The cuticle becomes a dark, thick-walled cyst.
       3. Anoxybiosis occurs with lack of oxygen.  The body swells and movement ceases.
15. Classification:
    • Class Eutardigrada: Lacks anterior cirri and lateral filaments
      • Order Macrobiotidae: Sucking pharynx with macroplacoids
      • Order Milnesiidae: Sucking pharynx without macroplacoids, mouth surrounded hy six papillae.
    • Class Heterotardigrada: Head with anterior cirri and lateral filaments, four separate but similar claws on each leg.
      • Order Echiniscoidea: Stout posterior legs not set off by cuticular folds.
    • Class Mesotardigrada: Six similar Long claws on each leg; one species, Thermozodium esakii is from Japanese hot springs.

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