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(Summary by Wesley R. Elsberry for Zoology 6207, Spring 1982)

  1. Definition of the phylum: Microscopic, protostomous coelomates displaying limited metamerism; possessing a chitinous cuticle, a distinct head, four pairs of ventral legs, a pair of stylets, a sucking pharynx, and a pair of Malpighian tubules.
  2. External features:
    1. Microscopic metazoans (0.05mm- 1.2mm)
    2. Bilaterally symmetrical, cylindrical hody
      1. Head: Bluntly rounded, contains mouth, carries eyespots, and may be equipped with sensory cirri.
      2. Body: Short, plump, cylindrical covered with a chitin- ous cuticle
      3. Legs: Four pairs of extensions of the ventrolateral body wall extending ventrally, ending in four to eight claws each.
    3. Color may result from pigment in the cuticle, dissolved materials in the body fluid, or from the contents of the digestive tract.
  3. Internal features:
    1. Chitinous cuticle is secreted by the underlying epidermis.
    2. Muscular system is composed of numerous muscle bands, each a single muscle cell, extending from one subcuticular point of attachment to another.
    3. A single, saccular gonad occupies the coelom. Other internal organs are suspended in a general hemocoel.
    4. The digestive system is composed of an anterior mouth wirh associated salivary glands and stylet apparatus, a sucking pharynx, an esophagus, a stomach or midgut, and a rectum or hindgut, emptying through the anus or cloaca.
      1. Mouth is located at anterior point of the head, and is stiffened by rings of cuticle
      2. The salivary glands, stylets, and sucking pharynx are known as the buccal apparatus. The stylets are extended to pierce plant or animal cell walls, then the sucking pharynx draws fluids into the digestive system. The salivary glands are believed to form new stylets at a molt.
      3. The esophagus ranges from long to short in length.
      4. Secretions in the midgut are acidic anteriorly and alka- line posteriorly. Ahsorption also occurs in the midgut.
      5. The hindgut empties through the anus, egestion is correlated with molting.
    5. Excretory system is composed of a dorsal excretory gland and a pair of Malpighian tubules.
    6. The nervous system is composed of a brain, a pair of longitudinal nerve strands, and four ventral ganglia.
      1. The brain is composed of two lateral lobes connected by two circumpharyngeal cords to a subpharyngeal ganglion.
      2. The ventral ganglia are united by the longitudinal nerve strands. Paired nerves from the brain and ganglia innervate the body. Long strands may have small terminal ganglia.
    7. Sense organs:
      1. Eyespots: a pair of red or black pigment cups on the head.
      2. Cirri: head cirri are believed to be tactile.
    8. The reproductive system is composed of a single saccular gonad in a coelomic pouch Iying dorsal to the digestive system.
      1. In the Eutardigrada, gonoducts open into the rectum.
      2. In Heterotardigrada, gonoducts open to the outside through a preanal gonopore.
      3. Males possess two vas deferentia, a swollen portion serving as a seminal vesicle.
      4. Females possess a single oviduct which passes to the right of the intestine.
    9. In accordance with the small size of tardigrades, there is no respiratory or circulatory system.
  4. Reproduction:
    1. Asexual: For some species of tardigrades, no males have been found. Parthenogenesis occurs in two fashions.
      1. In diploid tardigrades, meiosis distributes chromo- somes hetween the oocyte and the polar body. The chromosomes are duplicated, then a second mitotic divi- sion occurs, yielding a dipoid egg nucleus and polar body.
      2. In triploid tardigrades, oocytes are formed by mitotic divisions, yielding triploid eggs.
    2. Sexual:
      1. Females may lay eggs in the exuvium as they molt, after which males ejaculate spermatozoa inro the old cuticle. Fertilization is external, hut protected.
      2. As a female is molting, males may introduce spermatozoa into the gonopore or cloaca. The spermatozoa travel up the oviduct into the ovary where fertilization occurs.
  5. Embryology:
    1. Cleavage is total and apparently equal.
    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. Ecology and Physiology:
    1. Aquatic and semi-aquatic species in marine, freshwater, and damp terrestrial habitats. Typically found as part of interstitial communities, on filamentous algae, or inhabiting the surface films of mosses, lichens, and damp forest litter.
    2. Tardigrades feed on the fluids of plant and animal cells. Some tardigrades are entirely carnivorous.
    3. Tardigrades are prey for amoebas, nematodes, and other tardigrades.
    4. Most species appear to he eurythermal, tolerating temperatures from near freezing to upwards of 30 degrees 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 dessication. The animal contracts, loses water, and takes on a shriveled, wrinkled appearance. This "tun" may survive in this state from four to seven years. Animals have been recovered from this state after immersion in liquid helium, absolute alcohol, brine, and ether.
      2. Cysts may be formed when the animal undergoes stress in the form of damage, hunger, or abnormal environmental conditions. The animal withdraws into the cuticle and forms a dark, thick-walled cyst. The internal organs undergo some degeneration. The animal reconstitutes in favorable conditions.
      3. When deprived of oxygen, tardigrades will enter an anoxybiotic state: the animal swells, the body becomes turgid and movement ceases. This behavior is utilized to prepare specimens.
  7. Classification:
  8. Phylogeny:
    1. Enterocoelous development of coelomic pouches in common with deuterostomes.
    2. Characteristics shared with mites include four pairs of legs, indistinct segmentation, and piercing stylets.
    3. Tardigrades are believed to have many Aschelminthe affinities.
    4. Further study is required to authoritatively place the phylum Tardigrada.
  9. References:

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