Characters and Anatomy
Decapods, along with insects, scorpions and millipedes etc. belong to a
group called the Arthropoda, animals with a jointed exoskeleton. Arthropods
are made up from serially repeating segments known as metameres
(or somites). In decapods these
are numbered from 1 to 21, starting at the limbless front segment
(this only bears the animal's eyes and is known as the occular
metamere or acron). These metameres can then be grouped
into three main divisions (or tagmata):
With the exception of the first and last metameres, every segment bears
a pair of limbs. Arthropod workers have given these a range of
names, best demonstrated with the "keyboard crustacean"
- The head or the cephalon
- The thorax or the pereion
- The abdomen or the pleon
The major segments of a decapod. Abbreviations: a, Antennules; A, Antennae; m, Mandibles;
mx, Maxillules; Mx, Maxillae; T, Telson.
Altogether then, decapods
have 38 limbs, so why on earth does their name infer ten? Well,
the number of metameres in each tagma follows the caridean
facies. This states that, although all crustaceans have a
cephalon made up from five true segments (bearing the antennae,
antennules, mandibles, maxillae, and maxillules), those belonging
to the caridean facies will have a pereion made up from eight segments
and a pleon which is composed of seven (including the telson). However,
decapods can be identified within this, as the first three thoracipods
are modified into feeding appendages (or maxillepeds).
Externally it can be difficult to identify these because, along
with the mandibles, maxillae and maxillules, the maxillipeds are
often hidden within the carapace (see below)
|In this diagram, the colours
are coded the same as in the keyboard crustacean. These are also
used to demonstrate the evolutionary trends within the Decapoda...
so keep them in mind!
Decapods ('deca' = ten; 'pod' = foot) have five pairs of walking limbs (pereiopods)
on the back (orange) segment, making a total of ten, hence theire name.
Attached to the last
cephalic segment, and projecting over the thorax and cephalon,
the bivalved carapace forms a cephalothorax. The morphology
of this is very important to decapod classification and evolution
and if you would like to learn more about it, click
here. And, if you
would like to learn more about the structure of the leg or gills,