Arthropods

This is the largest and most successful of the animal phyla, with over 1 000 000 species identified to date and an estimated 1 000 000 000 000 000 000 (1 quintillion) individuals alive today. Some scientists estimate that there are over 200 million insects for each single person. That's just insects alone, and doesn't include the countless millions of spiders, crustaceans, millipedes, centipedes, etc that make up the phylum Arthropoda.

Arthropods are the most easily identifiable of all the animals, as they all share several things in common that no other animals share. First, all arthropods are protected by a hard exoskeleton (outer skeleton) composed of chitin (humans and other chordates have endoskeletons). This exoskeleton protects the organs, gives support for the body, and allows for efficient locomotion. However, because the exoskeleton cannot grow, it must periodically be shed to allow for the organism to grow.

All arthropods have segmented bodies divided into a head, thorax, and abdomen. In some cases, such as with the lobster, the head and thorax are fused together. This is called a cephalothorax. Arthropods also have jointed appendages.

Arthropods have a complex internal design, with ganglia for a brain and an organ that acts as a heart. They have a complete digestive and excretory system. They also have well-developed sensory organs, that include antennae, eyes, and in some cases internal ears. Respiration occurs through the body surface, gills, trachea, and/or book lungs. The oxygen is carried in an open circulatory system (humans have a closed circulatory system).

Arthropods are very diverse, and therefore can locomote in a wide variety of ways. They can walk, crawl, climb, hop, fly, glide, swim, skate, dive — almost every type of locomotion imaginable (other than drive!).

Arthropods can be found on every continent and in every ocean, from the harsh climate of Antarctica to the dryness of the desert to the dampness of the rainforests, and even to the darkness of the ocean's depths. They can be found in ponds, on plants, in and on other animals, and even in your own house!

Because this phylum is so large, there are numerous ways of classifying it. Below is the most widely-accepted 4-subphylum method of dividing this large phylum. While there are several other methods, including one in which Diplopoda and Chilopoda are divided into four separate classes, I believe this one to be the most accurate to date:

Subphylum Trilobita
This large subphylum consists of the trilobites, which are only known by the fossil record. This subphylum therefore is extinct.

Subphylum Chelicerata
Consists of arthropods that have 6 pairs of appendages, four of which are legs (or five in the case of the horseshoe crab). They lack antennae and mandibles. There are 3 classes:

Arachnida (spiders, ticks, mites, scorpions)
Merostomata (horseshoe crabs)
Pycnogonida (sea spiders)

Subphylum Crustacea
This subphylum contains 30 000 species of arthropods that lead mostly aquatic lives. They have two pairs of antennae and mandibles. There are 5 classes:

Branchiopoda (fairy shrimp, water fleas)
Cephalocarida (small crustaceans)
Malacostraca (isopods, amphipods, krill, crabs, shrimp, lobsters)
Maxillopoda (copepods, barnacles)
Remipedia (small, recently discovered crustaceans)

Subphylum Uniramia
This is the largest of the subphyla. These arthropods have one pair of antennae and one or two maxillae. They also have mandibles. Respiration occurs through the trachea, body surface and/or gills. There are 3 classes:

Chilopoda (centipedes)
Diplopoda (millipedes)
Insecta (insects)

THE OLD 4-CLASS TAXONOMIC SYSTEM

Arachnida (spiders, sea spiders, ticks, mites, scorpions)
Crustacea (crabs, lobsters, horseshoe crabs, krill, shrimp, barnacles, fairy shrimps, etc)
Insecta (insects)
Myriapoda (centipedes and millipedes)

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