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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 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, click here.