Arthropods

Arthropods (meaning "joint foot") are invertebrate animals that have an exoskeleton, instead of an endoskeleton like in vertebrates, a segmented body and paired jointed appendages. Arthropods include various known animals, such as arachnomorphs and insects. Their exoskeleton is made of chitin, sometimes mineralized with calcium carbonate that gives their tough surface, protecting arthropods from a potential external danger. Arthropods have a segmented body with appendages. The strong exoskeleton does not grow with the animal, so they will often shed and moult periodically its shell before they grow another. Insects are somewhat unique for possessing wings as a complement to their anatomy. They are among the most diversified animals in the world, being found virtually everywhere in the world. Over a million species have been described, which is already over 80% of all extant animal life described. They have ranged in size from microscopic proportions to the size of a small to medium-sized crocodile. Arthropods have an open circulatory system that runs haemolymph, instead of vertebrate blood, that accomodates the functioning of their internal organs. The nervous system follows a "ladder-like" sequence, in which they run through each segment and form paired ganglia in each one. The brain is formed by fusion of the ganglia on those segments. Many arthropods use the auxiliation of large compound eyes, large clusters of optical cells that serve as genuine tools to provide sight. They also have a wide range of chemical and mechanical sensors made out of their bristles, that come out of the cuticles. Terrestrial arthropods reproduce through internal fertilization, while aquatic species can perform internal or external fertilization, depending on the species. Arthropods evolved way back into the Cambrian or maybe even Precambrian ages. Arthropods are generally agreed, in their current form, as a monophyletic group, although the basis of the animal phylogeny is still in continuous discussion, and that might change things in the future. Almost all arthropods lay eggs. Recently born, arthropods can show themselves as miniature versions of the adults or as wormy grubs and larvae that eventually go through metamorphosis and become adults. Usually arthropod parental care is nonexistent, but that can differ in some species.