Phylum Arthropoda

Overview

Arthropods are easily the most diverse group of animals on the planet. The number of insects alone is greater than all the other animals combined. The fossil record shows a similar trend (at least for some groups): the number of trilobite species is greater than 10,000. Living arthropods include crabs, lobsters, shrimp, insects, barnacles, spiders, ticks, scorpians, garden sowbugs, horseshoe crabs, centipedes and millipedes. Fossil arthropods include trilobites and eurypterids, as well as many lesser known groups.

The success of arthropods can be attributed mainly by their ability to evolve rapidly to take advantage of changing environmental conditions and fill new or empty ecological niches.

Morphology

Arthropods typically have a bilaterally symmetrical body segmented into specialized regions. Each segment can bear at least one pair of appendages. These appendages were originally designed for walking, but every group has modified at least some of them for specialized uses: antenna for sensing, mouthparts for feeding, claws and pincers for food gathering or defense, gills for breathing, paddles for swimming, spines and stingers for protection, spinnerets for web building.

The segmentation of arthropods places them in close relationship with annelids, or segmented worms. In arthropods, however, a hard, jointed exoskeleton is formed over or around the soft body segments. This exoskeleton poses a special problem for growth. With a rigid shell enclosing it, the body cannot grow bigger. The solution is molting, in which the exoskeleton is periodically shed. The unprotected and vulnerable arthropod body then undergoes a period of rapid growth, after which a new exoskeleton is formed.

Classification

Kingdom Animalia
  Phylum Arthropoda
    Superclass Trilobitomorpha
      Class Trilobita

The Insects
Morphology

Insects are an easily recognized group of arthropods. Their body plan consists of a head, thorax and abdomen. No other groups of animals rivals the insects in terms of diversity. However, this diversity arises almost completely from modification of appendages rather than loss of them or loss of body segments.

The insect head consists of six segments. The first and third have no appendages and the second bears a pair of antennae. Segments four through six support modified appendages used in feeding. There are six segments on the thorax--each segment supporting a pair of legs. The first two thoracic segments also bear a pair of wings, but these are often reduced or lost. On the abdomen there are ten or 11 segments, though it may seem like fewer in some species because the segmentation is internal.

Classification

There are 25 to 30 recognized orders of insects (only a sampling of which is presented here) grouped into two subclasses based on whether or not the wings can be folded against the body:

    Superclass Hexapoda
      Class Insecta
        Subclass Palaeoptera
          Order Odonata         (dragonflies)
          Order Ephemeroptera   (mayflies)
        Subclass Neoptera
          Order Battoidea       (cockroaches)
          Order Isoptera        (termites)
          Order Orthoptera      (grasshoppers, crickets)
          Order Hemiptera       (true bugs)
          Order Homoptera       (aphids, whiteflies, cicadas)
          Order Hymenoptera     (ants, bees, wasps)
          Order Coleoptera      (beetles)
          Order Diptera         (flies)
          Order Lepidoptera     (butterflies, moths)

The Eurypterids
Morphology

Eurypterids are collectively called sea scorpians and consist of a head region called the prosoma and an abdominal region called the opisthosoma. They have one pair of kidney-shaped compound eyes in addition to a single pair of simple eyes called ocelli. Four pairs of walking legs are present. An additional pair of appendages is developed into large, prominent paddle-shaped structures presumably used for swimming. The opisthosoma consists of 13 segments, four of which housed gills. The last segment is developed into a spinelike structure called a telson.

Eurypterids are known from the Cambrian to the Permian Periods with peak diversity in the Silurian and Devonian.

    Superclass Chelicerata
      Class Merostomata
          Order Eurypterida
            Eurypterus lacustris 

The Decapods
Morphology

Decapods include lobsters, shrimp, crabs and crayfish and have a fossil record ranging from the Late Devonian to the present. Crustaceans in general have a head, thorax and abdomen. In decapods, the head and thorax are united to form the cephalothorax, which is often covered by a hard extension of the cuticle called the carapace.

The head has six segments, the first two bearing antennae. Appendages on the remaining segments are modified for feeding. The thorax has eight segments. The first three each have a pair of appendages called maxillipeds, which are modified as feeding structures. The name 'decapod' means 'ten legs'. These five pairs of legs are located behind the maxilliped-bearing segments. The last four pairs are usually developed as walking legs while the first are the large pincers (known as chelae) that the group is characterized by. Some pairs of walking legs may be lost. The abdomen has six segments; the last one is called the telson. In crabs, the abdomen is folded under the thorax and some segments are fused. The gills are protected in this arrangement allowing crabs to live in muddy environments that would clogs the gills of other decapods with sediment.

    Superclass Crustacea
      Class Ostracoda
      Class Malacostraca
        Subclass Eumalacostraca
          Order Decapoda
            Arpactocarcinus punctulatus 
            Paleonephrops browni


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