Phylum Arthropoda

Common Name: Insects and Chelicerates (Spiders, Scorpions, and Horseshoe Crabs)

Etymology: "Jointed-foot", from the Greek arthron a joint and pous for foot, in reference to the fact that the group is characterized by a chitinous exoskeleton with distinct joints on each limb.

Representative examples:

Click images to enlarge.
	Araneus marmoreus. A chelicerate.	Neolithodes grimaldii. The crab, a crustacean.	A freshwater prawn, or crayfish.

Symmetry: Bilateral Tissue Layers: 3 Coelom: Eucoelomate Digestive Tract: Complete Special Features: insect anatomy spider anatomy butterfly emerging, movie click to enlarge images

Arthropod anatomy, shown in a crustacean. butterfly metamorphosis, step-by-step

Additional Information:

Arthropods are a speciose (four-fifths of all known animals are arthropods) and morphologically diverse group of eucoelomate protostomes. They are characterized by a chitinous exoskeleton, jointed limbs, open circulatory systems, and a body plan divided into discrete segments with highly specialized appendages.

For the purposes of our lab, the Arthropoda will be divided into three subphyla: Chelicerata, Crustacea, and Uniramia. In common terms, these groups can be thought of as the spiders, the crabs, and the insects.

Chelicerates, represented in lab by the horseshoe crab (Limulus polyphemus), modify the basic arthropod body plan by specialization of some appendages and loss of the appendages associated with some body segments. The chelicerate body is divided into two main regions - an anterior cephalothorax (made up of 8 segments) and a posterior abdomen. The two most anterior segments of the cephalothorax lack appendages, and as a result the chelicerates have no antennae. The chelicerae, which give the group its name, are pointed appendages that grasp the food in place of the chewing mandibles most other arthropods have. They are first pair of appendages and are followed by five pairs of manipulatory pedipalps or locomotory legs. The legs on the cephalothorax are either uniramous or have a very reduced gill branch, and are adapted for walking or swimming. The appendages on the abdomen, in contrast, are either absent or are reduced to their gill branch. For more on horseshoe crabs, click here.

The Crustacea have 3 distinct body parts, head, thorax, and abdomen. They are also characterized by two pairs of antennae on the head and three pairs of mouthparts (two pairs of maxillae and 1 pair of mandibles). Their appendages are typically biramous (meaning they are split into two branches at the base, a robust locomotory branch and a smaller branch that may incorporate a gill-like structure), including the second pair of antennae (but not the first). Unlike the chelicerates, crustaceans generally have distinct abdominal appendages called swimmerets. Most crustaceans are aquatic with the exception of a few terrestrial forms like the common sow bug.

The phylum Uniramia are characterized by one pair of antennae and two pairs of mouthparts (single pairs of mandibles and maxillae). Their body forms and ecologies are diverse, though (in contrast to the crustaceans) most unirames are terrestrial. The Uniramia includes the insects and several smaller groups of related organisms. Note that most current classification schemes do not include the myriapods (centipedes and millipedes) in the Uniramia, instead placing them in their own subphylum, the Myriapoda (meaning "many-legged"). The evolutionary relationships between the four large subphyla mentioned are unclear. For a good article on this controversy see the links below.

Links:

• Horseshoe Crab Anatomy and More
• Arthropoda (Earthlife.net)
• The horseshoe crab ("Kabutogani", Samurai Warrior's Helmet Crab) in modern art.