Physical characteristics

Tardigrades are bilaterally symmetrical and vary in shape from cylindrical to extremely dorso-ventrally flattened. The majority of tardigrade species are white to translucent, but some terrestrial forms may exhibit strong colors such as yellow, orange, green, or red to olive-black. There are five distinct body segments, including a cephalic segment and four trunk segments, each bearing a pair of segmented legs with oblique- or cross-striated muscles. The terrestrial and limnic forms have reduced the segmentation in their stumpy legs that bear two to four claws, while the marine forms may have telescopic retractable legs, with up to 13 claws or four toes with complex claws. Other marine tardigrades have four to six toes with rod-shaped adhesive discs or round suction discs also inserted on the foot via toes.

The cuticle of tardigrades is very complex. Both the dorsal and ventral body cuticle may have segmental plates with different spines and appendages. The cuticle is frequently molted in juveniles and adults, much like in arthropods. In the beginning of the molting cycle, the tardigrade enters the simplex stage, which includes sheeting of stylets, stylet supports, buccal tubes, and pharyngeal cuticular rods, the so-called placoids. When the new cuticle is formed, the cuticle of the digestive system and toes/claws is also re-synthesized. The stylet apparatus is re-synthesized by two stylet glands ("salivary glands"), and cuticular claws and toes are formed in special claw glands in the legs.

The digestive system consists of three principal parts: the foregut (ectodermal origin), the midgut (mesodermal origin),

Colored scanning micrograph of a marine tardigrade (Macrobiotus sp.) showing four pairs of stumpy legs terminating in claws for clinging to sand or soil. (Photo by Andrew Syred/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.)

and the hindgut (ectodermal origin). The foregut is a very complex feeding structure that consists of a mouth cavity, a stylet apparatus, a buccal tube, and a tri-radiate pharynx with placoids. The two stylets and the stylet supporters are probably homologous with mouth limbs in arthropods.

The nervous system consists of a three-lobed brain, a subpharyngeal ganglion, and four ventral trunk ganglia. Paired eyespots may be present inside the forebrain. Tardi-grades lack respiratory organs, and gas exchange takes place through the epidermis. All tardigrades lack excretory proto-or metanephridia, which are common in many other invertebrates. Instead, eutardigrades have three Malpighian tubules at the junction between the mid- and hindgut. The Malpighian tubules may have both an excretory and os-moregulatory function. Heterotardigrades lacks these tubules, but some have segmental organs (coxal glands) that may have an excretory function.

The embryology of the tardigrades is still highly debated. Two theories exist: one theory postulates that the tardigrades have radial cleavage and an enterocoelic mode of coelom formation, whereas another theory suggests that the tardigrades have a modified spiral cleavage and schizocoelic mode of coelom formation. Schizocoely is also found among arthropods, which supports the close relationship between arthropods and tardigrades. The dispute about the tardigrade cleavage type has arisen because of disagreements among scientists about the observed cleavage pattern and cell fates. However, new and improved cell lineage studies appear to support the presence of radial cleavage.

Tardigrades are known to survive long periods of drying or freezing by cryptobiosis - a stage of latent life (ametabolic stage). In fact, it is only tidal tardigrades and the tardigrades that inhabit the interstitial water of mosses and lichens that are capable of cryptobiosis. There are four types of crypto-biosis: anhydrobiosis (dehydration), cryobiosis (very low tem perature), osmobiosis (water potential and strong variations in salinity), and anoxybiosis (lack of oxygen). Anhydrobiosis and cryobiosis are well investigated. In these forms of cryp-tobiosis, the tardigrades can survive from a few months to several years. Neither Osmobiosis nor anoxybiosis are accepted by all scientists as true forms of cryptobiosis, but this is perhaps only a question of insufficient investigations. Usually, the terrestrial species will die after only short time (a day) in water without oxygen, but the tidal tardigrade Echiniscoides sigismundi has survived up to six months in seawater without oxygen. This species is currently the only known species that has the capacities to enter all four types of cryptobiosis.

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