Approximately three-fourths of all animal species are in the phylum ARTHROPODA. The arthropod body organization must therefore be an extraordinarily successful one. Great structural are also implied by these numbers. The success of the phylum is obvious, whether measured by numbers of individuals or species, or by total mass, structural variety, adaptability, or evolutionary plasticity
The arthropod basic body type is characterized by:
1. Bilateral symmetry
2. Segmentation,
3. A hardened exoskeleton, usually chitinous,
4. Jointed appendages.
5. A strong tendency to ward agmosis (fusion of blocks of segments to form major regions –head, thorax, and abdomen) and toward heteronomous metamerism (formation of specialized segments and appendages)
6. Discontinuous growth, usually occurring immediately after shedding the exoskeleton (moltaing),
7. No distinct trochophorelike larval form in early development, such as in annelids and mollusks
8. Cephalization (increased sixe and speiclaisation of the brain and central nervous system
9. A tendency toward reduction of the coelom and formation of hemocoel; and
10. Retention of certain annelidike characters dorsal heart with ostia, nerve ring around esophagus, ventral ganglia paired in each segment but modified by fusion of ganglia and segments in more advanced forms).
The major groups of arthropods are classified according to their segmentation, tagmosis, and appendages. The first part to form embryologically, the head, contains the most specialized appendages and is probably the most useful key to relationships.
One of the most interesting aspects of arthropod biology is the extraordinary impact that the chitinous exoskeleton has had on the form, function, adaptability, and evolution of the group. This is discussed with respect to insect evolution as an illustration. Not only the term ARTHROPODA (meaning jointed feet,) but many other characters the manner of growth, circulatory and respiratory systems, size, musculature, and even habitat can be related to this tough, jointed, hollow skeleton. Limitations too- small size, short life span, restricted brain size -can
Although a neuplus larva is found is CRUSTACEA and a wormlike larva in higher insects, both are later stages in the embryological sequence be traced to the skeletal structure. Keep the relationship between exoskeleton and evlution of the phylum in mind during your survey of this enormous grou. Test its applicability as you become better acquainted with the examples reviewed here
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