The Arthropodan Phyla

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In zoological systematics, a phylum is a major level in the classification of the animal kingdom. A phylum comprises a set of species which share a set of adaptive features. In simple terms, this usually means that members of a phylum usually show a basic body plan or modifications of it. The phylum usually has one or more key features which are typical of that group but not found in other phyla, i.e. key features are  characteristic of the phylum. For example the Echinodermata (starfishes, sea urchins and relatives) all show rather bizarre pentamerous radial symmetry; no other phylum at all shows such a body organisation. Another of their key features is spiny skin, from which they derive their name ("echino" and "dermata"). Of course, some members of a phylum may be so highly adapted that it may be difficult to recognise the key features that make them part of that phylum (like the sea cucumbers - Holothuroidea - in the Echinodermata, which have a worm-like shape and soft skin!).

Importantly, member species of a phylum should possess one or more homologous features that are characteristic of the phylum and derived from a common ancestor.

Biological adaptations based on:

So great are the advantages of arthropodisation that it evolved AT LEAST THREE times (maybe more).

There are thus three extant phyla:- Crustacea, Uniramia, Chelicerata.
A fourth arthropod phylum is the Trilobitomorpha - now all its species are extinct.

In each of these phyla,  their biology is determined by adaptations linked to the possession of the rigid cuticle - but the hardening mechanisms are very different in the three extant groups. Thus, in my opinion this key feature (the cuticle) is analogous rather than homologous. It does not unify these three groups into a single phylum, but rather is the result of convergent evolution.
 

Crustacea (38,000 species)
cuticle hardened by calcium salts

shrimps, crabs, lobsters, etc..
evolved in the sea, now still mainly marine		 shrimp as example of Crustacea
butterfly as example of Uniramia

Uniramia (at least a million spp.)
cuticle hardened by quinone bonds

myriapods and insects
evolved on land, mainly terrestrial animals, though some secondary adaptations to aquatic life

Chelicerata (75,000 spp.)
cuticle hardened by disulphide links

scorpions, spiders, harvestmen, mites, etc...
evolved in the sea, now almost entirely terrestrial		 spider as example of Chelicerata


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