Arthropod

Arthropods are protostomes. There is a coelom, but it is reduced to a tiny
cavity around the reproductive and excretory organs, and the dominant body
cavity is a hemocoel, filled with blood which bathes the organs directly.
The arthropod body is divided into a series of distinct segments, plus a
presegmental acron which usually supports compound and simple eyes and a
postsegmental telson. These are grouped into distinct, specialized body
regions called tagmata. Each segment at least primitively supports a pair of
appendages.

The cuticle in arthropods forms a rigid exoskeleton, composed mainly of
chitin, which is periodically shed as the animal grows. The exoskeleton
takes the form of plates called sclerites on the segments, plus rings on the
appendages that divide them into segments separated by joints. This is in
fact what gives arthropods their name - joint feet - and separates them from
their very close relatives, the Onychophora and Tardigrada. At one point it
was considered that the different subphyla of arthropods had separate
origins from segmented worms, and in particular that the Uniramia were
closer to the Onychophora then to other arthropods. However, this is
rejected by most workers, and is contradicted by genetic studies.

Traditionally the Annelida have been considered the closest relatives of
these three phyla, on account of their common segmentation. More recently,
however, this has been considered convergent evolution, and the arthropods
and allies may be closer related to certain pseudocoelomates such as
roundworms that share with them growth by molting, or ecdysis. These two
possible lineages have been termed the Articulata and Ecdysozoa.

The classification of the arthropods varies somewhat from source to source.
There are five main subgroups: the Trilobita, Chelicerata, Myriapoda,
Hexapoda, and Crustacea, which may be variously ranked from subphyla to
classes, with various other taxa introduced above or below them and
corresponding changes in the ranks of their subgroups. Here we have followed
a "splitting" taxonomy, containing only generally accepted groups and
assigning them higher ranks.

Aside from these major groups, there are also a number of fossil forms,
mostly from the lower Cambrian, which are difficult to place, either from
lack of obvious affinity to any of the main groups or from clear affinity to
several. of them.