Arthropods

The main characteristic of representatives of this group of animals is the presence of a chitinous exoskeleton.

Arthropods are distributed almost worldwide and play a significant role everywhere. Insects, along with mammals and birds, are the dominant group of terrestrial animals. There are at least 1.5 million species of insects, possibly much more. It is estimated that their number ranges from 3–10 to 30 million. Although arachnids and myriapods have not reached the same diversity as insects, they are also widely spread. Finally, crustaceans inhabit almost all latitudes and depths of the world’s oceans. In some regions, small crustaceans are the dominant group of plankton.

Tardigrades known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals.

They live in diverse regions of Earth’s biosphere – mountaintops, the deep sea, tropical rainforests, and the Antarctic. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions – such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation – that would quickly kill most other forms of life. Tardigrades have survived exposure to outer space.

Onychophora, commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus, is a phylum of elongate, soft-bodied, many-legged animals. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. They prey upon other invertebrates, which they catch by ejecting an adhesive slime. Approximately 200 species of velvet worms have been described, although the true number of species is likely to be much greater than that.

The two extant families of velvet worms are Peripatidae and Peripatopsidae. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are all found south of the equator. It is the only phylum within Animalia that is wholly endemic to terrestrial environments, at least among extant members.