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