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invertebrate – Most animal life on Earth belongs to groups other than vertebrates – vertebrates represent less than 5% of known animal species currently living on our planet. There is no single unifying characteristic for invertebrates, although they do not usually posses internal skeletal structures; some invertebrate groups have extensive exoskeletons (e.g., arthropods, mollusks). Invertebrate animals dominate, and are often directly responsible for the creation of all aquatic and terrestrial ecosystems on the planet. While not considered primary producers (only plants are), their actions and existence make the life of other organisms possible. Examples of invertebrates include Oligochaeta (earthworms), Calcarea (sponges), Gastropoda (snails), Gordioidea (horsehair worms), Arachnida (spiders and relatives), Insecta (insects), Xiphosura (horseshoe crabs), and many, many others.
IUCN categories I-IV – In the IUCN classification system for protected areas, categories I-IV refer to higher levels of protection.
IUCN Red List – The IUCN Red List of Threatened Species provides information on the conservation status of species that have been globally evaluated for their probability of extinction using quantitative criteria.