Evolution: Arthropods

In reading Wonderful Life, Gould's book on the Cambrian explosion (some of the earliest multicellular life forms), I find Gould spending much of his time dwelling on arthropods. It's worth spending time on them, as that phylum (one step below Kingdom - Animal) constitutes most animals in the world: insects spiders and crustaceans in particular.

At first they look quite alien close up, but after a while, patterns emerge. And believe me, there are more alien-looking lifeforms around (eg the Cambrian creatures Opabinia or Hallucigenia). You can get used to arthropods...

Characteristics:

The prototypic body is constructed of a number of repeated segments. However, from that basic design, differentiation evolves. Typically, over time, the segments may become fused; further, different segments evolve different functionality - the head in particular, as well as the tail. Each segment begins with a pair of jointed appendages, one on each side of the segment. The appendages are jointed - arthropod means "jointed foot". Each appendage of the prototype is biramous - meaning each has two branches! - typically a leg for walking and a gill. However, this pattern can modify over evolutionary time, leaving at least some of the legs uniramous.

Arthropods have exoskeletons - that is, the skeleton is on the outside - although not what you'd consider bone.

The key to understanding them is how the segments fuse differently, how the appendages specialise, and how the exoskeleton can modify.

Often the segments are fused into three: for example, the head, thorax and abdomen of insects. Typically, the head is composed of several fused segments, and the attached appendages specialise, for example, as antennae or for feeding. With spiders, for example, the actual legs are on the head; in the posterior the walking legs have disappeared, and the gills have modified into oxygen-breathers.

Frequently, there are two pairs of appendages before the mouth and three after. Sometimes (eg in crabs) a carapace has formed over the body; sometimes that carapace is divided - bivalve.

The five major kinds are:

1. Trilobites: extinct marine creatures with three lobes across, and numerous segments

2. Chelicerates: spiders, mites and scorpions

3. Myriapods: centipedes and millipedes

4. Hexapods: mainly insects, typically six legs. By far the most numerous arthropods, with millions of species. Wikipedia aligns them with crustaceans, although Gould united Myriapods and Hexapods as 'Uniramia'

5. Crustaceas: mainly marine, mainly biramous; includes lobsters, crabs, barnacles, and many more.

Although this may seem an odd group, there are enough similarities to unites them; enough to posit a common ancestor.

References

Gould, Stephen Jay (1989): Wonderful Life: The Burgess Shale and the Nature of History, Penguin, London.

Wikipedia: Arthropod (extracted 12-Dec-2007)