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 toward 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