Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Introduction General features Size range Distribution and abundance Importance Form and function The exoskeleton and molting Muscles, appendages, and locomotion Digestive system and feeding Respiratory system Circulatory system Excretory system and water balance Nervous system and organs of sensation Reproductive system and life cycle Evolution and paleontology Classification Distinguishing taxonomic features Annotated classification Critical appraisal.
Load Previous Page. Evolution and paleontology The arthropods share many features with the phylum Annelida.ankolcofisi.tk
Adaptations of Arthropods
Load Next Page. More About. Animal Corner - What is an Arthropod? Arthropods share a common ancestor with polychaete worms, and may even be a direct descendant of polychaetes. But unlike other eucoelomate invertebrates, the arthropod coelom is greatly reduced in the adult animal. All arthropods have jointed appendages. This evolutionary innovation is probably the key to the stunning success of this diverse group. There are about 10 billion billion arthropods alive at any one time. There are over three times as many species of arthropods as there are of all other animals on Earth, and there may be millions more that we haven't even discovered.
Arthropods do everything with legs or modified legs. They walk, they swim, they creep and crawl, they use legs to sense with the antennae , to bite and sting with, and even to chew with. That's one reason arthropods look so alien when we see them up close. They chew sideways, and it's all done with legs. Their bodies are protected by an tough cuticle made of proteins and chitin , a polysaccharide with added nitrogen groups. A cuticle is a tough outer layer of non living organic material. The cuticle of arthropods acts as an exoskeleton. Most are very small, though a few lobsters reach up to a meter, and one giant crab grows to 3.
Fossil insects were also very large. Ancient dragonflies had wingspans of a foot or more. But living insects are uniformly small. Perhaps smaller insects were better at hiding or escaping from their many predators. Terrestrial arthropods remain small primarily because of the limitation imposed by their exoskeleton. A large insect would need such a thick exoskeleton to withstand its strong muscles that the weight of the cuticle would be too great for the animal to carry around.
The colonization of land by animals: molecular phylogeny and divergence times among arthropods
For a small animal, having your skeleton on the outside is as logical as having it on the inside. But it poses a fundamental problem for arthropods. They must shed their exoskeleton, or molt , in order to grow. The exoskeleton splits open. While the animal molts, it is especially vulnerable - just ask a plate of soft-shelled crabs! Arthropods have segmented bodies, like the annelid worms. These segments have become specialized, however, with one pair of jointed appendages added to each segment.
Among living arthropods, the millipedes most closely suggest what the ancestral arthropod might have looked like. Arthropod segments have also fused together into functional units called tagma.
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This process of segment fusion, or tagmosis , usually results in an arthropod body that consists of three major sections, a head, thorax, and abdomen. Sometimes the head and thorax are fused together into a cephalothorax. Each of these body sections still bear the appendages that went with it, though these appendages are often highly modified. Arthropods are very highly cephalized, often with intricate mouthparts and elaborate sensory organs, including statocysts , antennae, simple eyes and compound eyes.
Sensitive hairs on the surface of the body can detect touch, water currents, or chemicals.
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Their nervous systems are highly developed, with chains of ganglia serving various parts of the body, and three fused pairs of cerebral ganglia forming a brain. Aquatic arthropods respire with gills. Terrestrial forms rely on diffusion through tiny tubes called trachea. Trachea are cuticle-lined air ducts that branch throughout the body, and open in tiny holes called spiracles , located along the abdomen. Insects can open and close these spiracles, to conserve water that would otherwise be lost to evaporation from the open tubes.
Their reliance on diffusion for respiration is one of the reasons that insects are small. Arthropods excrete by means of malphigian tubules , projections of the digestive tract that help conserve water. Terrestrial forms excrete nitrogen as uric acid , as do birds. Their waste is nearly dry, a superb adaptation to life on land.
Arthropods have an open circulatory system, and separate sexes. Fertilization is usually internal, another adaptation for terrestrial life. Males and females often show pronounced sexual dimorphism. Order Orthoptera - grasshoppers, crickets, roaches. In chelicerates, the first pair of appendages are called chelicerae, and are modified to manipulate food.
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They are often modified as fangs or pincers. Chelicerates lack antennae. Horseshoe crabs have larvae that are very similar to trilobites, and they may be descendants of this long vanished group. Horseshoe crabs are nocturnal, feeding on annelids and molluscs. They swim on their backs, or walk upright on five pairs of walking legs.
They live in the deep ocean, migrating inshore in large numbers in the spring to mate on the beaches during moonlight and high tide - much like undergraduates on Spring Break. This very successful group of arthropods have four pair of walking legs 8 legs. The first pair of appendages are the chelicerae , and the second pair are pedipalps , appendages modified for sensory functions or for manipulating prey.
They are mostly carnivorous many mites are herbivores. Most secrete powerful digestive enzymes which are injected into the prey to liquify it. Once dissolved in its own epidermis, the prey is sipped like a root beer float. Order Scorpiones 2, sp. Scorpions date back to the Silurian, about mya, and may be the first terrestrial arthropods.