Other Ecdysozoa

Panarthropods (arthropods and their relatives) all have a leg tipped with a chitinous tarsal claw and use their appendages to manipulate their food before ingestion. Their cuticle is formed from alpha- chitin but is still thin and flexible in onychophorans that still have their muscles arranged in sheets. Tardigrade cuticle is a little more rigid and although their fleshy legs are extended by hydrostatic pressure banded muscle is used to retract them. All three groups (Onychophora, Tardigrada, Arthropoda) are, however, separated by large genetic distances, indicating an ancient divergence and obscuring their closer affinities.

1.TARDIGRADES

They are called 'water bears' because of their body shape. here are maybe 800 species living in mosses, soil, freshwater and the sea. The size of tardigrades is usually between 0.15 to 0.8 mm.

The most obvious part of the body is the four-segmented trunk with the four pairs of fleshy legs with tarsal claws or pads at their tips. Considered to have a hemocoel. Appendages are not jointed and the legs are extended by hydrostatic pressure and retracted by banded retractor muscles attached to the dorsal cuticle. Legs used to distinguish groups.

The almost Indistinguishable head includes an anterior mouth armed with stylets used to pierce food that is sucked into the digestive system.

 

The cuticle forms plates often sculpted into various patterns and spines and is moulted all at once but remains flexible enough to allow the body wall to function as a hydrostatic skeleton. The chitinous cuticle isn't waterproofed; restricting these animals to moist conditions. It's also an advantage; the surface is the main site for gas exchange.

Most of the body cavity is a hemocoel but there is a true coelom around the gonads.

The dormant stages, tuns, can withstand extreme conditions and undergo cryptobiosis.
Species have been known to survive temperatures as low as -272 ^oC for a few minutes which is close to absolute zero. Others have also survived temperatures as low as -200 for more than 20 days. Other species can go to the other extreme and survive temperatures as high as 125 ^oC well above the boiling point of water. still others survive doses of X-rays 250 times greater than that which would kill a mammal.

From Jonathan C Wright, Tardigrada Encyclopedia of Life Sciences DOI: 10.1038/npg.els.0004140

"Tardigrades are able to inhabit the ephemeral interstitial water of mosses and lichens owing to cryptobiosis. This extraordinary process enables the animals to survive complete loss of liquid water, although some ‘bound’ water remains associated with macromolecules even at extremely low water potentials. Hazards of dehydration include intracellular concentration of electrolytes, protein denaturation, and loss of the hydrophobic–hydrophilic orienting forces that maintain thermodynamic stability of membrane phospholipid bilayers. Cryptobiotic species replace electrolytes with compatible osmolytes that serve the additional function of preserving macromolecular structure. The most common of these ‘membrane protectants’ are sugars such as trehalose and mannose, and various polyols, particularly glycerol. As the moss cushion dries and water potential declines, tardigrades contract into a barrel-like tun sand drying of the cuticle initiates an abrupt decline in permeability. Thus protected from rapid desiccation, the tardigrade begins a rapid synthesis and mobilization of protectants that stabilize proteins and substitute for membrane-associated water over the succeeding hours."

https://www.youtube.com/watch?v=7W194GQ6fHI

https://www.earthlife.net/inverts/tardigrada.html

 

There are separate sexes, although males are rare. Females usually reproduce through parthenogenesis. Meiotic (haploid) eggs are produced seasonally in some populations, but it is not known what factors control the switch between mictic (sexual) and amictic (parthenogenetic) generations. Growth involves periodic shedding of the cuticle (ecdysis) and continues throughout life. Egg laying and defecation usually only occur during molting

Sexes are normally separate, but some species can be parthenogenetic.

Tardigrades are eutelic, meaning there is a fixed number of cells in the body of an adult of any given species, this is normally around 40,000 cell

 

2. Onychophora

Does this clade provide a glimpse at a fossil arthropod? Cambrian fossils show that abundant marine relatives of the Onychophora flourished in the seas 520 million years ago. The most famous of these is Hallucigenia, from the Burgess Shale.

http://evolution.berkeley.edu/evolibrary/article/_0_0/cambrian_13

https://www.youtube.com/watch?v=9-Z9Ssgb0Kg

 

Onychophorans hunt isopods, termites and small molluscs.

Onychophoran predators shoot a proteinaceous glue from large "slime glands"openings at the tips of the oral papillae. Some Onychophorans are herbivores or omnivores and may use their "glue" as defense. In predators, once prey is trapped, jaws are used to grasp and cut prey. Some species will inject salivary secretions that start to digest prey, so the semi-liquid tissues can be sucked into its mouth.

https://www.youtube.com/watch?v=DEnDQojDRPU

The Onychophoran cuticle is molted in patches, rather than all at once allowing the hemocoel to act as complex hydrostatic skeleton (how body changes in length contribute to forward locomotion not fully understood) using the circular and longitudinal muscle lining the body wall. Body is considered a hemocoel.

Appendages are not jointed and legs are extended by hemocoelic pressure. Although muscles in legs direct forward locomotion, their activity moves the body pass a stationary point in the extended limb tip. Body interestingly remains rigid during movement but undergo continual changes in width and length. Most propulsion is thought to be a function of a complex interaction of the muscle in the limbs and the pressure generated by the hemocoel.

Paired antennae, jaws, and the oral papillae that secrete the slime used to capture prey, represent the three head segments. Body covered by thin layer of chitin and skin is covered in tiny papillae. Papillae are made up of overlapping scales-makes the skin hydrophobic and appear "velvety." Papillae are connected to sensory nerve cells and sensitive to touch, smell, and air currents. A thick layer of muscular tissue is located beneath the epidermis.

The brain lies above the esophagus and connects to a ladder-like nervous system running the length of the body.

A pair of eyes is connected to the protocerebrum.

Onychophorans use a tracheal system and spiracles open near the base of the legs. Their cuticle is very permeable and so they are restricted to moist environments. Excretory system resembles that of crustaceans but rather than excreting aqueous ammonia or urea, both of which require the elimination of water at significant rates to prevent them accumulating to toxic concentrations, onychophorans excrete uric acid.

Onychoporans have separate sexes and fertilization occurs both internally and externally depending on species. Onychophorans usually exhibit sexual dimorphism. That is, males and females are not the same size. As is most often the case in animals that are sexually dimorphic, the female is larger than the male. A few species are oviparous, meaning they lay the eggs outside of the body. Most others retain the eggs inside the body, within the mother's uterus, but not attached to the mothers body. A few Onychophorans keep the eggs within the females uterus, and they are maintained in contact with a sort of placenta. They gain nutrients slowly throughout their development from the mother's system.

Their affinities with the Arthropoda are now seldom challenged and are well supported by such characters as the chitin cuticle, hemocoel and arthropod-type hemocyanins. Molecular sequence data from 12S ribosomal ribonucleic acid (rRNA) provides further evidence that onychophorans are closely related to arthropods. Fossils exist that are 300 million years old.