Invertebrates (1) are animals that do not have a backbone or a bony skeleton. Scientists have identified close to one million living species of invertebrates but this represents only a small fraction of the total number of invertebrates alive today. Scientists estimate that there are perhaps as many as 30 million species of living invertebrates—although in all likelihood we will never know the exact number of invertebrate species with whom we share this planet. What we do know is the following staggering statistic: that more than 97 percent of all animal species alive today are invertebrates. The vertebrates, the group to which we humans and all other mammals, birds, reptiles, amphibians, and fishes belong, are far outnumbered by our invertebrate cousins.
There are more than thirty groups of invertebrates. Some of the more commonly known groups include arthropods, cnidarians, echinoderms, molluscs, segmented worms and sponges. Some lesser known groups include ctenophores, platyheominthes, nematodes, rotifers, braciopods, lorciferans and bryozoans.
The diversity of invertebrates can be overwhelming for anyone attempting to understand this group of animals. But by looking for variations on simple themes, scientists have been able to classify and characterize this overwhelmingly large group of animals into more comprehensible subgroups. For example, the concept of a ground plan (also called a bauplan) provides a blueprint of each group of invertebrates that unites them based on simple, observable characteristics. Arthropods, for instance, are known for having segmented bodies and jointed appendages. Echinoderms are known for their pentaradial symmetry. Cnidarians are known for their radially symmetrical and the tentacles that encircle their mouth.
Since invertebrates are so diverse, discussing general characteristics common to all invertebrates is nearly impossible and not that valuable. The one trait that unites invertebrates is the absense of a verebral column (or in other words, the lack of a backbone). But this shared characteristic provides little useful insights into invertebrate biology. Grouping animals based on whether or not they have a backbone is as arbitrary as dividing all animals into those that can fly (such as most insects, birds and bats) and those that cannot fly (such as springtails, reptiles, amphibians, fishes and mammals other than bats) and drawing distinctions based on that.
With invertebrates, it's most useful to learn the characteristics of the main groups, such as arthropods, cnidarians, echinoderms, molluscs and such. At that level of classification, more can be said about their shared adaptations, anatomy, and evolutionary history than can ever be said for all invertebrates as one big group. The main things to know about invertebrates is that they lack a backbone and bony skeleton, there are an overwhelming number and diversity of invertebrates, and to dig deeper into our understanding of invertebrates, it is best to examine the various invertebrate groups individually.
The key characteristics of invertebrates include:
- no backbone
- earliest animals were invertebrates
- more than 97 percent of all living species are invertebrates
Invertebrates are classified within the following taxonomic hierarchy:
Animals > Invertebrates
Some of the better-known groups of invertebrates include:
- Arthropods (Arthropoda) - There are more than one million known arthropods species and many millions more that have yet to be named. Scientists estimate there may be as many as 30 million species of arthopods (most of which are insects). Members of this group include centipedes, millipedes, spiders, mites, horseshoe crabs, scorpions, insects and crustaceans. Arthropods are bilaterally symmetrical and have a segmented body, an exoskeleton, jointed appendages and numerous pairs of legs and specialized limbs.
- Cnidarians (Cnidaria) - There are about 9,000 species of cnidarians alive today. Members of this group include jellyfish, corals, sea anemones and hydras. Cnidarians are radially symmetrical and have a gastrovascular cavity with a single opening that is surrounded by tentacles.
- Echinoderms (Echinodermata) - There are about 6,000 species of echinoderms alive today. Members of this group include star fish, sea lilies, feather stars, brittle stars, sea cucumbers and sea urchins. Echinoderms are pentaradially symmetrical and have an endoskeleton composed of calcareous ossicles.
- Molluscs (Mollusca) - There are about 100,000 species of molluscs alive today. Members of this group include bivalves, gastropods, cephalopods, tusk shells, and several other groups. Molluscs have soft bodies that consist of three basic parts: a foot, a visceral mass and a mantle.
- Segmented Worms (Annelida) - There are about 12,000 species of segmented worms alive today. Members of this group include earthworms, ragworms and leeches. Segmented worms are bilaterally symmetrical and their body consists of a head region, a tail region and a middle region of numerous repeated segments
- Sponges (Porifera) - There are about 10,000 species of sponges alive today. Members of this group include calcarious sponges, demo sponges and glass sponges. Sponges are primitive multi-celluar animals that have no digestive system, circulatory system or nervous system.
Find out more: The Basic Invertebrates Groups
(1) The term invertebrate does not correspond to a taxonomic group of animals in the way mammals or birds or reptiles do. Instead, the term invertebrate provides scientists with an informal way of referring to a wide variety of animal groups that share a single common characteristic: the lack of a backbone or bony skeleton. Simply put, invertebrates include all animal species that are not vertebrates.
Borrell, B. One-fifth of invertebrate species at risk of extinction. Nature. 03 Sep 2012. DOI: 10.1038/nature.2012.11341.
Ruppert E, Fox R, Barnes R. Invertebrate Zoology: A Functional Evolutionary Approach. 7th ed. Belmont CA: Brooks/Cole; 2004. 963 p.