Arthropods are the largest grouping of animals all of which have jointed legs and an exoskeleton made of chitin.
- Describe the morphology of arthropoda
- Arthropods include the Hexapoda (insects), the Crustacea (lobsters, crabs, and shrimp), the Chelicerata (the spiders and scorpions), and the Myriapoda (the centipedes and millipedes).
- Arthropods have a segmented body plan that contains fused segments divided into regions called tagma.
- Arthropods have an open circulatory system and can use book gills, book lungs, or tracheal tubes for respiration.
- tagma: a specialized grouping of arthropodan segments, such as the head, the thorax, and the abdomen with a common function
- malpighian tubule: a tubule that extends from the alimentary canal to the exterior of the organism, excreting water and wastes in the form of solid nitrogenous compounds
- spiracle: a pore or opening used (especially by spiders and some fish) for breathing
The name “arthropoda” means “jointed legs” (in the Greek, “arthros” means “joint” and “podos” means “leg”); it aptly describes the enormous number of invertebrates included in this phylum. Arthropods dominate the animal kingdom with an estimated 85 percent of known species included in this phylum; many arthropods are as yet undocumented. The principal characteristics of all the animals in this phylum are functional segmentation of the body and presence of jointed appendages. Arthropods also show the presence of an exoskeleton made principally of chitin, which is a waterproof, tough polysaccharide. Phylum Arthropoda is the largest phylum in the animal world; insects form the single largest class within this phylum. Arthropods are eucoelomate, protostomic organisms.
Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphyla: Trilobitomorpha (trilobites, all extinct), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustaceans (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs). Trilobites are an extinct group of arthropods found chiefly in the pre-Cambrian Era that are probably most closely related to the Chelicerata. These are identified based on fossil records.
Trilobite fossil: Acadoparadoxides, possibly A. briareus, a large trilobite from about 500 million years ago from Morocco, North Africa (Middle Cambrian)
A unique feature of animals in the arthropod phylum is the presence of a segmented body and fusion of sets of segments that give rise to functional body regions called tagma. Tagma may be in the form of a head, thorax, and abdomen, or a cephalothorax and abdomen, or a head and trunk. A central cavity, called the hemocoel (or blood cavity), is present; the open circulatory system is regulated by a tubular, or single-chambered, heart. Respiratory systems vary depending on the group of arthropod. Insects and myriapods use a series of tubes (tracheae) that branch through the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae. Other organisms use variants of gills and lungs. Aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates use book gills. The book lungs of arachnids (scorpions, spiders, ticks, and mites) contain a vertical stack of hemocoel wall tissue that somewhat resembles the pages of a book. Between each of the “pages” of tissue is an air space. This allows both sides of the tissue to be in contact with the air at all times, greatly increasing the efficiency of gas exchange. The gills of crustaceans are filamentous structures that exchange gases with the surrounding water.
Book gills: The ventral side of a horseshoe crab showing the book gills located near the telson (tail). These gills flap back and forth bringing oxygen to the blood.
Groups of arthropods also differ in the organs used for excretion. Crustaceans possess green glands while insects use Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body of nitrogenous waste. The cuticle is the covering of an arthropod. It is made up of two layers: the epicuticle, which is a thin, waxy, water-resistant outer layer containing no chitin; and the chitinous procuticle, which is beneath the epicuticle. Chitin is a tough, flexible polysaccharide. In order to grow, the arthropod must shed the exoskeleton during a process called ecdysis (“to strip off”); this is a cumbersome method of growth. During this time, the animal is vulnerable to predation.