lting 1.5 Internal organs 1.6 Sense

Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions.

The versatility of the arthropod modular body plan has made it difficult for zoologists and paleontologists to classify them and work out their evolutionary ancestry, which dates back to the Cambrian period. From the late 1950s to late 1970s, it was thought that arthropods were polyphyletic, that is, there was no single arthropod ancestor. Now they are generally regarded as monophyletic. Historically, the closest evolutionary relatives of arthropods were considered to be annelid worms, as both groups have segmented bodies. This hypothesis is now largely rejected, with annelids and molluscs forming the superphylum Lophotrochozoa. Many analyses support a placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall however, the basal relationships of Metazoa are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated.

Arthropods contribute to the human food supply both directly as food, and more importantly as pollinators of crops. Some specific species are known to spread severe disease to humans, livestock, and crops.
Contents

    1 Description
        1.1 Diversity
        1.2 Segmentation
        1.3 Exoskeleton
        1.4 Moulting
        1.5 Internal organs
        1.6 Senses
    2 Reproduction and development
    3 Evolution
        3.1 Last common ancestor
        3.2 Fossil record
        3.3 Evolutionary family tree
    4 Classification
    5 Interaction with humans
    6 See also
    7 Notes

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