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Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 m (8 ft 2 in). [ 100 ] The oldest known arachnid is the trigonotarbid Palaeotarbus jerami , from about 420 million years ago in the Silurian period. [ 101 ] [
Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs . The scientific study of animals is known as zoology , and the study of animal behaviour is known as ethology .
a Beroe ovata, b unidentified cydippid, c "Tortugas red" cydippid, d Bathocyroe fosteri, e Mnemiopsis leidyi, and f Ocyropsis sp. [17]. Among animal phyla, the ctenophores are more complex than sponges, about as complex as cnidarians (jellyfish, sea anemones, etc.), and less complex than bilaterians (which include almost all other animals).
The four extant species of hyena, clockwise from upper left: spotted hyena (Crocuta crocuta), brown hyena (Parahyaena brunnea), aardwolf (Proteles cristata), and striped hyena (Hyaena hyaena)
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[31] [32] Beavers can swim at 8 km/h (5.0 mph); [25] only their webbed hind feet are used to swim, while the front feet fold under the chest. [31] On the surface, the hind limbs thrust one after the other; while underwater, they move at the same time. [33] Beavers are awkward on land but can move quickly when they feel threatened.
Birds are a group of warm-blooded vertebrates constituting the class Aves (Latin:), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.