Invertebrates
Scientific Classification
- Domain Eukaryota
- Clade Obazoa
- Kingdom Animalia
Invertebrates are animals that lack a vertebral column or backbone. Unlike vertebrates, which possess a vertebral column and constitute the subphylum Vertebrata, invertebrates are an informal group that includes sponges, cnidarians, echinoderms, flatworms, arthropods, mollusks, and annelids, among others.
Almost 97% of all animal species are invertebrates, and the IUCN estimates that over 1,300,000 invertebrate species have been described worldwide.[1][2] Of these, insects alone account for around 1,000,000 species, making them the largest invertebrate group.
These animals are cosmopolitan in distribution, occupying every habitat, including land, seas, and freshwater bodies.
Invertebrates vs Vertebrates
The following table outlines the general characteristics of invertebrates, in comparison with those of vertebrates.
| Characteristic | Invertebrates | Vertebrates |
|---|---|---|
| Vertebral Column | Absent | Present |
| Size | Smallest: Myxozoans — 10-25 µm (0.001-0.0025 cm)[3] Largest: Giant squid — 42 ft 8 in (13 m)[4] |
Smallest: Microhylid frog — 0.3 in (7.7 mm) Largest: Blue whale — 108 ft (33 m) |
| Body Symmetry | Can be asymmetrical, bilaterally symmetrical, or radially symmetrical | Most are bilaterally symmetrical, but very few are asymmetrical |
| Body Cavity (Coelom) | Can be absent (acoelomate), partial (pseudocoelomate), or complete (coelomate) | Present and always complete (coelomate) |
| Body Covering | Either have a hard outer covering (chitinous cuticle/ calcaerous shell/ spiny ossicles) on their skin or a moist, naked body | The skin is typically modified into scales, feathers, or hair |
| Level of Body Organization | Ranges from the cellular and tissue level to the organ-system level | Always organ-system level |
| Circulatory System | Mostly open (body fluids bathe organs directly), but a few have a closed system (blood vessels supply organs) | Always closed |
| Respiratory System | Breathe through gills, tracheae, skin, or lung-like organs | Breathe primarily through gills and lungs, though some use both skin and lungs |
| Digestive System | Can be incomplete (with a single opening for ingestion and excretion) or complete (with a separate mouth and anus) | Always complete |
| Nervous System | Can be decentralized (nerves form a simple network) and centralized (nerves concentrated into specific nerve centres) | Always centralized |
| Reproduction | Mostly sexual, with some groups being asexual | Almost entirely sexual, but a few can reproduce asexually |
Classification
In 1973, French naturalist Jean-Baptiste Lamarck coined the term ‘invertebrates’ as an umbrella term encompassing all animals lacking a vertebral column. However, it does not refer to any particular taxon as opposed to Vertebrata.
There are currently over 30 phyla of invertebrates.
- Porifera (Sponges)
- Cnidaria (Jellyfish and sea anemones)
- Ctenophora (Comb jellies)
- Platyhelminthes (Flatworms)
- Nematoda (Roundworms)
- Mollusca (Mollusks, like snails, slugs, bivalves, and tusk shells)
- Annelida (Annelids, such as bristleworms, earthworms, and leeches)
- Arthropoda (Arthropods, such as insects, springtails, crustaceans, chelicerates, and myriapods)
- Echinodermata (Echinoderms, such as starfish, sea urchins, sea lilies, brittle stars, and sea cucumbers)
- Hemichordata (Hemicordates, such as acorn worms)
- Chordata (All chordates outside Subphylum Vertebrata,namely, lancelets and tunicates)
- Xenacoelomorpha (Acoels)
- Placozoa (Placozoans)
- Mesozoa (Mesozoans)
- Nemertea (Ribbon worms)
- Gnathostomulida (Jaw worms)
- Gastrotricha (Gastrotrichs)
- Rotifera (Rotifers)
- Chaetognatha (Arrow worms)
- Nematomorpha (Horsehair worms)
- Priapulida (Penis worms)
- Kinorhyncha (Mud dragons)
- Loricifera (Loriciferans)
- Acanthocephala (Spiny-Headed worms)
- Cycliophora (Cycliophorans)
- Entoprocta (Entoprocts)
- Bryozoa (Bryozoans or moss animals)
- Phoronida (Horseshoe worms)
- Brachiopoda (Brachiopods)
- Sipuncula (Peanut worms)
- Tardigrada (Tardigrades or water bears)
- Onychophora (Velvet worms)
The three phyla Priapulida, Kinorhyncha, and Loricifera are grouped under the informal group Scalidophora, while Bryozoa, Phoronida, and Brachiopoda are placed under another informal group, Lophophorata.
These animals first appeared at least around 665 million years ago, as revealed by fossils from the Trezona Formation in South Australia that are interpreted as early sponges.[5] Later, around 453 million years ago, the different invertebrate groups began diverging from one another.