Crayfish are members of the Phylum Arthropoda - meaning "jointed foot". They are in the Subphylum Crustacea. This subphylum ranges in size from microscopic copepods to the king crab - with a leg span of over 12 feet.
Crayfish and close relatives like lobsters are sometimes called "decapods" - meaning that they have ten feet, or five pairs of legs. Crayfish actually have four pairs of "walking legs". The other pair of legs are the chelipeds - commonly refered to as "pinchers".
The body of the crayfish is covered by a two-part exoskeleton made of chitin. Chitin contains a high percentage of calcium, similar to your fingernails.
The small appendages along the ventral side of the abdomen are the swimmerets. The first pair (closest to the abdomen) can be used to determine the sex of a crayfish. In males, this pair of swimmerets is used to deposit sperm into the oviducts of the female. They are larger and harder than the others. In females, all the swimmerets are soft and used to carry the fertilized eggs and newly hatched young.
Each leg has a gill attached. The movement of the legs helps circulate water over these gills. Notice that there is thin, transparent layer of exoskeleton separating the gills from the body cavity. This division allows water to cover the gills without getting inside the body cavity.
The gonads are located just above the heart. In the female, they appear as a pair of tubular structures along each side that combine behind the heart into a single mass, the ovary. In the male, the testes are small tightly coiled white tubes.
The crayfish has a two-part stomach. The "cardiac" stomach is a large sac-like structure in which food is stored. Posterior to it is the smaller "pyloric" stomach, where most of the digestion occurs. Digestive glands are located on each side of the pyloric stomach which produce digestive enzymes.
The excretory structures, green glands, are located at the base of each antenna. These act as the kidneys of the crayfish.
The open circulatory system of the crayfish has no veins. Two arteries leave the heart, the abdominal aorta (toward the posterior) and the anterior dorsal aorta (toward the anterior). Blood flows from the dorsal arteries to capillaries and then into tissue spaces called sinuses which function as veins.
The ventral nervous system of the crayfish a similar to (but more complex than) that of the earthworm. The "brain" is a mass of nerve ganglion just in front of and above the esophagus. Notice the very fine thread-like nerves connecting the brain to the eyes, antannae, and antennules. Two nerves lead from the brain, around the exophagus, and combine to form the ventral nerve cord which runs to the end of the abdomen. There is a small bundle of nerves, ganglion, in each segment of the body. Pairs of nerves come off each of these to appendages and internal organs that lie within each segment.