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The Jointed Legged Animals, Phylum Arthropoda:

Insects, Crabs, Shrimps, Lobsters, Krill, Copepods, Mysids, Stomatopods...

By Bob Fenner

Periclimenes holthuisi

Anilocra laticaudata an isopod parasite of fishes. Here on a Butterflyfish host in Grand Turks, in the tropical West Atlantic. 

Classification: Taxonomy, Relation With Other Groups

If you absolutely loathe biological classification and all of it's obscure terminology, you're going to hate the crustaceans. There are so many "split-hairs" amongst it's groups that science types have taken to making new infra-, sectional-, you-name-it categories to sub-divide all the groups. I guess that's what you get with such a collection of diverse organisms and excellent fossil history. Anyway it won't hurt my feelings if you skip on down to the next subject heading.

The Phylum Arthropoda (="jointed legs").

With the insects, trilobites, spiders, horseshoe crabs, scorpions, ticks, mites, sea spiders, centipedes, millipedes, (whew!) and a few other various and sundry groups, the subphylum Crustacea helps make up the largest phylum of life (Arthropoda); with some three quarters of a million described species. They all share 1) Metamerism: a division of the body into definite segments. 2) A pair of appendages per segment (at least in the primitive condition). 3) A nervous system with a dorsal, anterior brain, ventral nerve cord and ganglionic swellings in each segment. 4) Similar embryology/development. 5) Chitinous exoskeletons that cover the entire body; growth through periodic molting. 6) A muscular system that moves the animal by pulling on the exoskeleton.

Enough of the higher taxonomy of the group! A synopsis of the arthropod classes:

Subphylum Trilobita. The fossil trilobites.

Subphylum Chelicerata. Chelicerae for mouthparts...

Class Merostomata. The living horseshoe crabs and fossil eurypterids (some were big predators).

Class Arachnida. The scorpions, harvestmen, spiders mites and ticks.

Class Pycnogonida. The sea spiders.

Subphylum Uniramia

Class Insecta. The insects.

Class Chilopoda. The centipedes.

Class Diplopoda. The millipedes.

Bibliography/Further Reading:

Marine Hitchhiker/Critter ID (Maughmer, Toonen, Tompkins)

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