Phylum Arthropoda

A Suit of Armor
Sometime, somewhere in the Precambrian era, a major milestone in the evolution of life on earth was passed. The soft cuticle in an ancestor of animals we now call arthropods was stiffened by deposition of additional amounts of protein and an inert polysaccharide called chitin. The cuticular exoskeleton was some protection against predators and other environmental hazards, and it conferred on its possessors a formidable array of other selective advantages. Of course, a suit of armor could not be uniformly stiff; the animal would be as unable to move as the rusted tin woodsman in the Wizard of Oz. Stiff sections of cuticle were separated from each other by thin, flexible sections, which formed joints. The cuticular exoskeleton had enormous evolutionary potential. Jointed extensions on each metamere became appendages.

Once the stiffened cuticle evolved, or perhaps concurrently with it, many other changes were necessary in the bodies of the protoarthropods. Growth required a sequence of cuticular molts controlled by hormones. Coelomic compartments lost their hydrostatic skeletal function, causing a regression of the coelom and its replacement with an open system of sinuses (hemocoel). Motile cilia were lost. These changes and others are called “arthropodization.” Some zoologists argue that all changes in arthropodization follow from development of a cuticular exoskeleton. If several different ancestors had independently evolved a cuticular exoskeleton, then they independently would have evolved the identical suite of characters we associate with arthropodization. The huge phylum we call Arthropoda would be in reality polyphyletic. However, we agree with other zoologists who feel that the weight of evidence still supports single-phylum status.

Position in Animal Kingdom

1. Shared derived characters suggest that both annelids and arthropods evolved from a line of coelomates, segmented protostomes with spiral cleavage and mosaic development.
2. Evolution of a hard cuticular exoskeleton was followed or accompanied by arthropodization, which included loss of intersegmental septa; development of hemocoel and loss of closed circulatory system; jointed appendages; conversion of body-wall muscles to insert on cuticle.
3. Like annelids, arthropods have conspicuous metamerism, but their somites have greater variety and more grouping for specialized purposes; specialization of appendages, with pronounced division of labor, results in greater variety of action.

Biological Contributions

1. Cephalization becomes more pronounced, with centralization of fused ganglia and sensory organs in the head.
2. Compared with annelids, somites are more specialized for a variety of purposes, forming functional groups (tagmosis).
3. The presence of paired jointed appendages diversified for numerous uses produces greater adaptability.
4. Locomotion is by extrinsic limb muscles, in contrast to the body musculature of annelids. Striated muscles confer rapidity of movement.
5. Although chitin is found in a few groups other than arthropods, its use is better developed in arthropods. The cuticular exoskeleton, containing chitin, is a great innovation, making possible a wide range of adaptations.
6. The tracheae represent a breathing mechanism more efficient than that of most invertebrates.
7. The alimentary canal shows greater specialization by having, in various arthropods, chitinous teeth, compartments, and gastric ossicles.
8. Behavioral patterns are much more complex than those of most invertebrates, with a wider occurrence of social organization.
9. Many arthropods have well-developed protective coloration and protective resemblances.

Phylum Arthropoda
Phylum Arthropoda (ar-throp´o-da) (Gr. arthron, joint, + pous, podos, foot) is the most extensive phylum in the animal kingdom, composed of more than three-fourths of all known species. Approximately 900,000 species of arthropods have been recorded, and probably at least as many more remain to be classified. However, based on surveys of insect fauna in the canopy of rain forests, many estimates of yet undescribed species are much higher. Arthropods include spiders, scorpions, ticks, mites, crustaceans, millipedes, centipedes, insects, and some others. In addition, there is a rich fossil record extending to the very late Precambrian period.

Arthropods are eucoelomate protostomes with well-developed organ systems, and they share with annelids the property of conspicuous metamerism.

Arthropods have an exoskeleton containing chitin, and their primitive pattern is that of a linear series of similar somites, each with a pair of jointed appendages. However, the pattern of somites and appendages varies greatly in the phylum. There is a tendency for somites to be combined or fused into functional groups, called tagmata (sing., tagma), for specialized purposes; appendages are frequently differentiated and specialized for pronounced division of labor.

Few arthropods exceed 60 cm in length, and most are far below this size. The largest, a Japanese crab Macrocheira (Gr. makros, large, + cheir, hand), spans approximately 4 m; the smallest is a parasitic mite Demodex (Gr. demos, body, frame, + dex, a wood worm), which is less than 0.1 mm long.

Arthropods are usually active, energetic animals. They utilize all modes of feeding—carnivorous, herbivorous, and omnivorous—although most are herbivorous. Most aquatic arthropods depend on algae for their nourishment, and the majority of land forms live chiefly on plants. In diversity of ecological distribution, the arthropods have no rivals.

Although arthropods compete with humans for food and spread serious diseases, they are essential in pollination of many food plants, and they also serve as food, yield drugs and dyes, and produce products such as silk, honey, and beeswax.

Arthropods are more widely and more densely distributed throughout all regions of the earth than are members of any other phylum. They are found in all types of environment from low ocean depths to very high altitudes, and from the tropics far into both north and south polar regions. Different species are adapted for life in the air; on land; in fresh, brackish, and marine waters; and in or on the bodies of plants and other animals. Some species live in places where no other animal could survive.

Why Have Arthropods Achieved Such Great Diversity and Abundance?
Arthropods have achieved a great diversity, number of species, wide distribution, variety of habitats and feeding habits, and power of adaptation to changing conditions. In the following discussion we briefly summarize some structural and physiological patterns that have been helpful to them.

1. A versatile exoskeleton: Arthropods possess an exoskeleton that is highly protective without sacrificing mobility. This skeleton is the cuticle, an outer covering secreted by the underlying epidermis. The cuticle is made up of an inner and thicker procuticle and an outer, relatively thin epicuticle. The procuticle is divided into an exocuticle, which is secreted before a molt, and endocuticle, which is secreted after molting. Both layers of the procuticle contain chitin bound with protein. Chitin is a tough, resistant, nitrogenous polysaccharide that is insoluble in water, alkalis, and weak acids. Thus the procuticle not only is flexible and lightweight but also affords protection, particularly against dehydration. In some crustaceans the chitin may form 60% to 80% of the procuticle, but in insects it is probably not more than 40% (the remainder being protein). In most crustaceans the procuticle is also impregnated with calcium salts, which reduce its flexibility. In the hard shells of lobsters and crabs, for instance, this calcification is extreme. The outer epicuticle is composed of protein and lipid. The protein is stabilized and hardened by chemical cross-linking, adding further protection. Both the procuticle and epicuticle are laminated, that is, composed of several layers each (see Figure 31-1) The cuticle may be soft and permeable or may form a veritable coat of armor. Between body segments and between the segments of appendages it is thin and flexible, creating movable joints and permitting free movements. In crustaceans and insects the cuticle forms ingrowths (apodemes) that serve for muscle attachment. It may also line foregut and hindgut, line and support the trachea, and be adapted for biting mouthparts, sensory organs, copulatory organs, and ornamental purposes. It is indeed a versatile material.
   
   The nonexpansible cuticular exoskeleton does, however, impose important restrictions on growth. To grow, an arthropod must shed its outer covering at intervals and grow a larger one—a process called ecdysis, or molting. Arthropods molt four to seven times before reaching adulthood, and some continue to molt after that. An exoskeleton is also relatively heavy and becomes proportionately heavier with increasing size, thereby limiting ultimate body size.


2. Segmentation and appendages for more efficient locomotion. Typically each somite bears a pair of jointed appendages, but this arrangement is often modified, with both segments and appendages specialized for adaptive functions. Limb segments are essentially hollow levers moved by internal muscles, most of which are striated for rapid action. The jointed appendages have sensory hairs and may be modified and adapted for sensory functions, food handling, swift and efficient walking, and swimming.


3. Air piped directly to cells. Most terrestrial arthropods have a highly efficient tracheal system of air tubes, which delivers oxygen directly to the tissues and cells and makes a high metabolic rate possible. This system also tends to limit body size. Aquatic arthropods breathe mainly by some form of gill that is quite efficient.


4. Highly developed sensory organs. Sensory organs are found in great variety, from the compound (mosaic) eye to those accomplishing touch, smell, hearing, balancing, chemical reception. Arthropods are keenly alert to what happens in their environment.


5. Complex behavior patterns. Arthropods exceed most other invertebrates in complexity and organization of their activities. Innate (unlearned) behavior unquestionably controls much of what they do, but learning also plays an important part in the lives of many of them.


6. Limiting intraspecific competition through metamorphosis. Many arthropods pass through metamorphic changes, including a larval form quite different from the adult in structure. Larval forms often are adapted for eating food different from that of adults and occupy a different space, resulting in less competition within a species

Characteristics of Phylum Arthropoda

1. Bilateral symmetry; metameric body divided into tagmata consisting of head and trunk; head, thorax, and abdomen; or cephalothorax and abdomen
2. Jointed appendages; primitively, one pair to each somite, but number often reduced; appendages often modified for specialized functions
3. Exoskeleton of cuticle containing protein, lipid, chitin, and often calcium carbonate secreted by underlying epidermis and shed (molted) at intervals
4. Complex muscular system, with exoskeleton for attachment, striated muscles for rapid actions, smooth muscles for visceral organs; no cilia
5. Reduced coelom in adult; most of body cavity consisting of hemocoel (sinuses, or spaces, in the tissues) filled with blood
6. Complete digestive system; mouthparts modified from appendages and adapted for different methods of feeding
7. Open circulatory system, with dorsal contractile heart, arteries, and hemocoel (blood sinuses)
8. Respiration by body surface, gills, tracheae (air tubes), or book lungs
9. Paired excretory glands called coxal, antennal, or maxillary glands present in some, homologous to metameric nephridial system of annelids; some with other excretory organs, called malpighian tubules
10. Nervous system of annelid plan, with dorsal brain connected by a ring around the gullet to a double nerve chain of ventral ganglia; fusion of ganglia in some species; welldeveloped sensory organs
11. Sexes usually separate, with paired reproductive organs and ducts; usually internal fertilization; oviparous or ovoviviparous; often with metamorphosis; parthenogenesis in some.