Arthropoda

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Arthropod, animal with a hard, outer skeleton and a jointed body and limbs. Arthropods make up a phylum of invertebrates that includes insects, such as ants, beetles, and butterflies; crustaceans, such as lobsters, shrimps, and crabs; and arachnids, including scorpions, spiders, and ticks. In terms of sheer numbers and the variety of niches they fill, arthropods are the most successful animals on Earth. More than one million arthropod species have been identified—more than 20 times the number of known fish, amphibian, reptile, bird, and mammal species combined. This figure is considered a low estimate of the phylum's actual size because many arthropod species have yet to be discovered and documented. Some scientists suggest the number of arthropod species in tropical forests alone may approach six million to nine million.

Traditionally, living arthropods are classified among three subphyla: the Chelicerata, Crustacea, and Unirama. Trilobites, a fourth subphylum, flourished during the Cambrian and Ordovician periods of the Paleozoic Era (570 million to 435 million years ago) but became extinct by the mid-Permian Period (about 250 million years ago), for reasons that are not fully understood.

Chelicerata, named for their chelicerae, or specialized mouthparts, include scorpions, spiders, ticks, mites, and horseshoe crabs. These creatures have six pairs of appendages, one or two body regions, and lack wings and antennae. Most live on land, although a few are aquatic.

With a few exceptions, the Crustacea are aquatic, occupying both freshwater and saltwater niches. Familiar crustaceans include shrimps, lobsters, and crabs. All crustaceans have ten limbs, three body regions, and two pairs of antennae. Many of the smallest crustaceans are zooplankton—drifting or weak-swimming animals that form the base of most marine food webs.

The Unirama include centipedes, millipedes, and insects. Centipedes have wingless, multisegmented bodies, with one pair of legs on each body segment. The bodies of millipedes are also wingless and multisegmented but have two pairs of legs per segment. Insects have three pairs of legs, one pair of antennae, and may be wingless or have one or two pairs of wings. The majority of insects are land-dwellers, although some live in freshwater or marine environments.

Scientific classification: Arthropods make up the phylum Arthropoda, which is divided into three living subphyla: Chelicerata, Crustacea, and Unirama.

Types of Invertebrates

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Arthropoda -> Largest and most diverse invertebrate phylum characterized by animals with jointed limbs, a segmented body, and an exoskeleton made of chitin. Arthropods are abundant and successful in almost all habitats. The exoskeleton is divided into plates that enhance flexibility and movement. Periodic molting of the exoskeleton permits growth. Arthropods have a complex brain and nervous system. Many arthropods have a compound eye made up of numerous light-sensitive parts. Reproduction is sexual. – Examples: Ants, beetles, butterflies, lobsters, shrimp, crabs, scorpions, spiders, ticks

Ectoprocta or Bryozoa -> Small, mostly marine animals that resemble simplified horseshoe worms, with a lophophore surrounding the mouth. They live in colonies, attaching to the sea bottom or seaweed. Bryozoans are hermaphroditic and reproduce asexually (by budding) and sexually. – Examples: Moss animals

Entoprocta or Kamptozoa -> Small, mostly marine animals with a globular body mounted on a stalk. A lophophore surrounds both the mouth and anus. Entoprocts live in colonies. They reproduce both asexually and sexually. – Examples: Urnatella, Pedicellina

Brachiopoda -> Marine animals that resemble clams except that their shells form on the top and bottom of the animal, while clam shells form on the left and right of the animal. The shell attaches to the ocean bottom, rocks, or other objects by means of a cordlike stalk. Brachiopods reproduce sexually. – Examples: Lamp shells

Echinodermata -> Marine animals distinguished by their radial symmetry in which the body can be divided into five parts arranged around a central axis. They have internal skeletons composed of calcareous ossicles with projecting spines that give the body surface a bumpy appearance. They use body appendages called processes for feeding and locomotion. Reproduction is sexual and some have a larval form. – Examples: Sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers

Cycliophora -> Discovered in 1995, these parasites live in the mouthparts of certain lobsters. They have a characteristic saclike body with a bell-shaped mouth structure called a buccal funnel. Only one species has been identified, and it displays alternation of generations, with both asexual and sexual components of the life cycle. – Example: Single species identified as Symbion pandora

Loricifera -> Marine sediment dwellers with a protective structure called a lorica, consisting of plates that surround the animal's trunk. The mouth and head retract into the lorica for protection. Reproduction is sexual and little is known about the life cycle, although a larval form has been identified. – Examples: Nanaloricus mysticus

Onychophora -> Terrestrial, caterpillar-like animals found only in the tropics and southern hemisphere. The soft body is covered by a flexible cuticle that periodically molts. The head region contains a pair of antennae and clawlike mandibles. They have from 14 to 43 pairs of legs, depending upon the species and gender. Reproduction is sexual. – Examples: Velvet worms

Chaetognatha -> Torpedo-shaped marine animals with fins that enable them to swim with rapid, dartlike movements as well as gliding and floating motions. Movable hooks on their heads are used to capture prey. These animals are hermaphrodites and reproduction is sexual. – Examples: Arrow worms

Hemichordata -> Simple, wormlike marine animals with a primitive notochord and a system of gills. They use a distinctive proboscis to capture food and also to aid in locomotion. Reproduction is sexual and some have a larval form that resembles the larvae of echinoderms. – Examples: Acorn worms