Arthropods include an incredibly diverse group of taxa such as insects, crustaceans, spiders, scorpions, and centipedes. There are far more species of arthropods than species in all other phyla combined, and the number of undescribed species in the largest assemblage of arthropods, the insects, probably numbers in the tens of millions. Members of the phylum have been responsible for the most devastating plagues and famines mankind has known. Yet other species of arthropods are essential for our existence, directly or indirectly providing us with food, clothing, medicines, and protection from harmful organisms.

 

The systematic relationships of arthropod groups is not fully understood, which is not surprising given the size and diversity of the phylum.

 

A number of important characteristics are shared by most members of this phylum. Arthropods are bilaterally symmetrical protostomes with strongly segmented bodies. Segmentation affects both external and internal structure. Some segments are fused to form specialized body regions called tagmata; these include the head, thorax and abdomen, and the process and condition of fusion is called tagmosis. The body is covered with an exoskeleton made up primarily of chitin (a polysaccharide) in a protein matrix; lipids, other proteins, and calcium carbonate also play a role. Primitively, each body segment bears a pair of segmented (jointed) appendages; in all living arthropods, many of these appendages are dramatically modified or even lost. Arthropods generally grow by molting their exoskeletons in a process called ecdysis. Movement of appendages is controlled primarily by a complex muscular system, divided into smooth and striated components as in chordates. Cilia are not present. Most arthropods have a pair of compound eyes and one to several simple ("median") eyes or ocelli; either or both kinds of eyes may be reduced or absent in some groups. Arthropods are eucoelomate with the coelom formed by schizocoely, but the volume of the coelom is much reduced and usually restricted to portions of the reproductive and excretory systems. Most of the body cavity is an open "hemocoel," or space filled loosely with tissue, sinuses, and blood. The circulatory system is open and consists of a heart, arteries, and the open spaces of the hemocoel. The gut is complete. Respiration takes place through the body surface, and/or by means of gills, tracheae, or book lungs. The nervous system is annelid-like, with a brain (=cerebral ganglion) and a nerve ring surrounding the pharynx that connects the brain with a pair of ventral nerve cords. These cords contain numerous ganglia. Most arthropds are dioecious and have paired reproductive organs (ovaries, testes). Fertilization is internal in most but not all groups. Most lay eggs, and development often proceeds with some form of metamorphosis.

 

---

 

Sources:

 

Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.

 

Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.

 

Pearse, V., J. Pearse, M. Buchsbaum, and R. Buchsbaum. 1987. Living Invertebrates. Blackwell Scientific Publications, Palo Alto, Ca.

 

 

Crustaceans conserve minerals when moulting by absorbing calcium carbonate from their shell before it is shed.

       
  "The external shell gives the crustaceans the problem it gave the trilobites. It will not expand and since it completely encloses their bodies, the only way they can grow is to shed it periodically. As the time for the moult approaches, the animal absorbs much of the calcium carbonate from its shell into its blood. It secretes a new, soft wrinkled skin beneath the shell. The outgrown armour splits and the animal pulls itself out, leaving it more or less complete, like a translucent ghost of its former self. Now its skin is soft and it must hide, but it grows fast and swells its body by absorbing water and stretching out the wrinkles of its new carapace. Gradually this hardens and the animal can again venture into a hostile world." (Attenborough 1979:58)
  Learn more about this functional adaptation.

 

Limbs of crustaceans allow movement along several planes by clustering two or three joints on a limb, each working in a different direction.

       
  "The limbs, which are tubular and jointed, are operated by internal muscles. These extend from the end of one section, along its length, to a prong from the next section which projects across the joint. When the muscle contracts between these two attachment points, the limb hinges. Such joints can only move in one plane, but crustaceans deal with that limitation by grouping two or three on a limb, sometimes close together, each working in a different plane so that the end of the limb can move in a complete circle." (Attenborough 1979:58)
  Learn more about this functional adaptation.

 

The joints of some arthropods have two degrees of bending freedom (up-down and left-right) thanks to two 1-degree bending joints found at right angles to each other.

         
  "Bending both up-down and left-right…Arthropods gain two degrees of bending freedom by putting two 1-degree bending joints next to each other, each oriented at a right angle to the other…The classic work on such cases was done by S.M. Manton, in the 1950s and 1960s; as put, with a long list of references, by Wainwright et al. (1976), 'The accounts of her researches in this field constitute a monument in the study of mechanical design of the most mechanically diverse group of organisms that have ever lived.'" (Vogel 2003:403)
  Learn more about this functional adaptation.

 

The cuticle of arthropods provides rigid protection via its composite structure.

     
  "What we're calling rigid materials includes a lot of familiar biological items. There's arthropod cuticle, a composite of chitin fibers in some proteinaceous material, with the addition of calcium carbonate salt in the larger crustaceans. In many instances, the fibers are arranged in sheets, each with a specific orientation, rather like plywood." (Vogel 2003:305)
  Learn more about this functional adaptation.