Phylum: Arthropoda

The phylum Arthropoda is the largest and most varied in the animal kingdom. It includes well over one million described species. This represents approximately three-quarters of all known biological organisms, living or extinct. Countless arthropods remain undescribed (not yet named and studied), and the actual number of living species could be as high as ten million or more. Some of the more well known arthropods include insects, crustaceans, and spiders, as well as the fossil trilobites. Arthropods are found in virtually every known marine (ocean-based), freshwater, and terrestrial (land-based) ecosystem, and vary tremendously in their habitats, life histories, and dietary preferences.

Arthropods are characterized by the possession of a segmented body with appendages on each segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton that is made out of chitin, a polysaccharide. Periodically, an arthropod sheds this covering when it moults. This covering makes arthropods less prone to dehydration.

The success of the arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion. The muscle system is more or less assisted by hydraulics originated from the blood pressure created by the heart. The hydraulic system in spiders is especially well developed.

Most arthropods breathe through a tracheal system (exceptions are arthropods like sea spiders, Pauropoda, some thysanurans and some arachnids like many mites who breathe through their body surface instead); a potential difficulty considering that the skeletal structure is external and covers nearly all of the body. Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialized for gas exchange. Insects and most other terrestrial species have tracheal systems: air sacs leading into the body from pores, called spiracles, in the epidermis cuticle. Others use book lungs, or gills modified for breathing air as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing membrane. The gill chambers in terrestrial crabs have sometimes two different structures; one that is gilled used for breathing underwater, another especially adapted to take up oxygen (a pseudolung).

Arthropods have an open circulatory system. Haemolymph containing haemocyanin, a copper-based oxygen-carrying protein, is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are potstones. There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a haemocoel, filled with haemolymph that bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a presegmental acron, which usually supports compound and simple eyes and a postsegmental telson. These are grouped into distinct, specialized body regions called tagmata. Each