The Invertebrates

Life on Earth evolved from single-celled oragnisms that lived in the oceans around 3,500  million years ago. Jellyfish, simple worms and other multicellular animals later evolved in the seas followed by creatures with hard outsides, such as shellfish and trilobites. These are the primitive arthropods.

These arthropods later became the first land animals, emerging from the oceans as scavengers about 420 million years ago. The co-evolution of land plants provided resources for numerous different arthropod species, the most successful being the insects.

Today, invertebrates (animals without a backbone) make up  97% of living animals. Vertebrates (animals with a backbone) make up the remainding 3%. Within the invertebrates, the huge group Arthropoda contains the majority of species - over 1 million.

To enable us to catalogue the huge diversity of species on Earth, we rely on taxonomy: a process of defining groups of biological organisms on the basis of shared characteristics and giving names to those groups.

Each Kingdom (e.g. Kingdom: Animalia) is divided into Phylum and there are thought to be about 35 extant (living) phyla (multiple of phylum), which are further divided into Class, which are themselves divided into Order, which are divided into Family, divided into Genus and then the highest level of classification, Species.

In school, I was taught to remember the order as: Kettle Parts Can Often Fuse Giving Smoke. Bizarre, but I do still remember it!

Invertebrate Phyla 

Phylum Placozoa Placozoa 1

Phylum Porifera Sponges 10 000

Phylum Cnidaria Sea Anemones, Jellyfishes 9 500

Phylum Ctenophora Comb Jellies 90

Phylum Platyhelminthes Flat Worms 25 000

Phylum Mesozoa Mesozoa 50

Phylum Nemertina Ribbon Worms 900

Phylum Gnathostomulida Sand Worms 80

Phylum Gastrotricha Gastrotrichs 400

Phylum Rotifera Rotifers 2000

Phylum Nematoda Nematodes 80 000

Phylum Nematomorpha Horsehair Worms 240

Phylum Kinorhyncha Spiny-crown Worms 100

Phylum Loricifera Brush Heads 10

Phylum Acanthocephala Spiny-headed Worms 1 000

Phylum Cycliophora Cycliophorans 1

Phylum Entoprocta Marine Mats 150

Phylum Ectoprocta Bryozoans 5 000

Phylum Phoronida Phoronans 10

Phylum Brachiopoda Lamp Shells 335

Phylum Mollusca Slugs, Snails, Squid. 110 000

Phylum Priapulida Phallus Worms 16

Phylum Sipuncula Peanut Worms 320

Phylum Echiura Spoon Worms 140

Phylum Annelida Earthworms, Ragworms 9 000

Phylum Tardigrada Water Bears 700

Phylum Pentastoma Tongueworms 70

Phylum Onychophora Peripatus 80

 Phylum Arthropoda Insects, Crabs, Spiders etc. 1 000 000

Phylum Pogonophora Beard Worms 120

Phylum Echinodermata Starfish and allies 6 000

Phylum Chaetognatha Arrow-worms 110

Phylum Hemichordata Hemichordates 90

Phylum Chordata Chordates (Tunicates, Mammals, Birds etc.) 50 000