Arthropods

Arthropods are very diverse, but all have the following general characteristics:

They are segmented invertebrates with bilateral body symmetry.

All have an exoskeleton, which is a hard outer shell made of chitin (same as fungus walls).

All have jointed appendages. Appendages are structures such as antennae or legs that grow out of the body of an animal. These appendages are adapted for a wide variety of uses.

All have highly advanced nervous and sensory systems. Many can detect odors far better than humans and other mammals can.

Most have specialized mouthparts adapted for various feeding methods.

Main classes of Arthropods

Crustaceans

  • Most are aquatic and have gills. The only land dwelling crustaceans are pill bugs and sow bugs, which live in rotting logs and under rocks.
  • Some crustaceans have three body sections, some have only two.
  • Crustaceans have two pairs of antennae for sensing.
  • Most have five pairs of walking legs. The first pair is often claws.

Examples: crabs, lobsters, crayfish, barnacles and shrimp.

Arachnids

  • Have only two body segments: cephalothorax and abdomen.
  • Have six pairs of appendages. Generally 4 pairs are legs, one pair is pinchers or fangs and one pair are pedipalps, which are like short legs for grabbing stuff.
  • The most well known arachnids are spiders. Only about 12 species of spider in the world are poisonous enough to be deadly to humans. In the USA only black widow and brown recluse spiders are potentially deadly. The large, scary looking garden spiders in your yard or house are not deadly!
  • Breathe with book lungs, which are a little like layered gills adapted for air instead of water.

Other arachnids: ticks, mites and scorpions.

Horseshoe Crabs

  • Are not actually crabs and aren’t even crustaceans.
  • Are considered living fossils because they have not changed (evolutionarily) in 500 million years, which is before dinosaurs were around.

Centipedes and Millipedes

  • Centipedes have 15-181 segments, depending on the species.
  • Centipedes have one pair of legs per segment.
  • Centipedes have poison and if a human is bitten it is like a bee sting or worse. A few are poisonous enough to require hospital treatment. Large centipedes have been known to capture and eat mice!
  • Millipedes have two pairs of legs per segment and do not bite.

Insects

  • There are more different species of insects (750,000) than all other animals combined.
  • Insects have three body sections: head, thorax and abdomen.
  • Breathe with a complex network of branching tubes called tracheal tubes. The opening of tracheal tubes to the outside air is called a spiracle.
  • They have six legs (three pairs) attached to the thorax and many have one or two pairs of wings.
  • Most have a larva or caterpillar stage when young, then change (metamorphosize) into the adult form.
  • Although many are pests, they are extremely important for pollinating flowering plants. Without insect pollination, most fruits would never develop.

Echinoderms

Echinoderm means “spiny skin”.

Echinoderms:

  • Have radial symmetry
  • Have endoskeletons (skeletons inside their body) made of calcium carbonate.
  • Move by using water vascular systems (hydraulics) in their appendages.
  • Have simple nervous systems.
  • Have mouth, stomach and intestines.
  • All live in the ocean.

Classes of echinoderms:

  1. Sea stars (starfish)—most have 5 rays (legs), some have up to 40. Bottom of rays are covered with tube feet, controlled by a water vascular system. They are aggressive predators that eat shellfish. Strong enough to pull open clamshells.
  1. Brittle stars—look somewhat like sea stars but with longer, thinner legs. Legs break off easily (can even be shed on purpose), but can grow back in a few weeks. By far the fastest moving of all echinoderms. Feed on dead organisms on the bottom.
  1. Sea urchins and sand dollars—have no legs. Sand dollars are round and flattened with short spines. Sea urchins are ball-shaped with long spines. Many sea urchins have poison in their spines. Urchins (along with snails) are considered very important grazers of algae, preventing overabundance of algae in the intertidal zone.
  1. Sea cucumbers—look like a vegetable shaped blob. Crawl on the bottom with tentacles and tube feet. Feed on dead organisms found on the bottom.
  1. Feather Stars and Sea Lilies—as with horseshoe crabs, these are living fossils. Instead of hunting, these echinoderms are filter feeders. The arms of feather stars are different from typical starfish in that they are highly branched, like a feather or dust mop. This allows them more surface area to sweep plankton from the water.