|Cladistically included but traditionally excluded taxa|
Familiar examples of invertebrates include arthropods (insects, arachnids, crustaceans, and myriapods), mollusks (chitons, snail, bivalves, squids, and octopuses), annelid (earthworms and leeches), and cnidarians (hydras, jellyfishes, sea anemones, and corals).
The majority of animal species are invertebrates; one estimate puts the figure at 97%.
Many invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata.
The word "invertebrate" comes from the Latin word vertebra, which means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate.
The jointed aspect of vertebra is derived from the concept of turning, expressed in the root verto or vorto, to turn.
The prefix in- means "not" or "without".
In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord.
That would at least circumscribe the Chordata.
However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or perhaps bauplan.
Despite this, the concept of invertebrates as a taxon of animals has persisted for over a century among the laity, and within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata.
The following text reflects earlier scientific understanding of the term and of those animals which have constituted it.
According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external.
They include hugely varied body plans.
Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms.
Number of extant species
By far the largest number of described invertebrate species are insects.
|Invertebrate group||Latin name||Image||Estimated number of
The trait that is common to all invertebrates is the absence of a vertebral column (backbone): this creates a distinction between invertebrates and vertebrates.
The distinction is one of convenience only; it is not based on any clear biologically homologous trait, any more than the common trait of having wings functionally unites insects, bats, and birds, or than not having wings unites tortoises, snails and sponges.
Being animals, invertebrates are heterotrophs, and require sustenance in the form of the consumption of other organisms.
With a few exceptions, such as the Porifera, invertebrates generally have bodies composed of differentiated tissues.
There is also typically a digestive chamber with one or two openings to the exterior.
Morphology and symmetry
A minority, however, exhibit no symmetry.
One example of asymmetric invertebrates includes all gastropod species.
The origin of gastropod asymmetry is a subject of scientific debate.
They often have one claw much larger than the other.
If a male fiddler loses its large claw, it will grow another on the opposite side after moulting.
Sessile animals such as sponges are asymmetrical alongside coral colonies (with the exception of the individual polyps that exhibit radial symmetry); alpheidae claws that lack pincers; and some copepods, polyopisthocotyleans, and monogeneans which parasitize by attachment or residency within the gill chamber of their fish hosts).
Neurons differ in invertebrates from mammalian cells.
Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH.
Learning and memory using nociceptors in the sea hare, Aplysia has been described.
Mollusk neurons are able to detect increasing pressures and tissue trauma.
Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods.
One type of invertebrate respiratory system is the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues.
The distribution of spiracles can vary greatly among the many orders of insects, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it.
Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion.
Unlike vertebrates, insects do not generally carry oxygen in their haemolymph.
Note that despite being internal, the tracheae of arthropods are shed during moulting (ecdysis).
Like vertebrates, most invertebrates reproduce at least partly through sexual reproduction.
These fuse to form zygotes, which develop into new individuals.
Others are capable of asexual reproduction, or sometimes, both methods of reproduction.
Social interaction is particularly salient in eusocial species but applies to other invertebrates as well.
Insects recognize information transmitted by other insects.
The term invertebrates covers several phyla.
One of these are the sponges (Porifera).
They were long thought to have diverged from other animals early.
They lack the complex organization found in most other phyla.
Their cells are differentiated, but in most cases not organized into distinct tissues.
Sponges typically feed by drawing in water through pores.
Some speculate that sponges are not so primitive, but may instead be secondarily simplified.
The Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.
Both have distinct tissues, but they are not organized into organs.
As such, they are sometimes called diploblastic.
The Echinodermata are radially symmetric and exclusively marine, including starfish (Asteroidea), sea urchins, (Echinoidea), brittle stars (Ophiuroidea), sea cucumbers (Holothuroidea) and feather stars (Crinoidea).
All these organisms have a body divided into repeating segments, typically with paired appendages.
In addition, they possess a hardened exoskeleton that is periodically shed during growth.
The Nematoda or roundworms, are perhaps the second largest animal phylum, and are also invertebrates.
Roundworms are typically microscopic, and occur in nearly every environment where there is water.
A number are important parasites.
These groups have a reduced coelom, called a pseudocoelom.
Another phylum is Platyhelminthes, the flatworms.
These were originally considered primitive, but it now appears they developed from more complex ancestors.
The Rotifera or rotifers, are common in aqueous environments.
Also included are two of the most successful animal phyla, the Mollusca and Annelida.
The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches.
These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.
Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.
Among lesser phyla of invertebrates are the Hemichordata, or acorn worms, and the Chaetognatha, or arrow worms.
Classification of invertebrates
Invertebrates can be classified into several main categories, some of which are taxonomically obsolescent or debatable, but still used as terms of convenience.
Each however appears in its own article at the following links.
- Sponges (Porifera)
- Comb jellies (Ctenophora)
- Hydras, jellyfishes, sea anemones, and corals (Cnidaria)
- Starfishes, sea urchins, sea cucumbers (Echinodermata)
- Flatworms (Platyhelminthes)
- Earthworms and leeches (Annelida)
- Insects, arachnids, crustaceans, and myriapods (Arthropoda)
- Chitons, snails, bivalves, squids, and octopuses (Mollusca)
- Roundworms or threadworms (Nematoda)
The earliest animal fossils appear to be those of invertebrates.
665-million-year-old fossils in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia have been interpreted as being early sponges.
Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago.
Around 453 MYA, animals began diversifying, and many of the important groups of invertebrates diverged from one another.
Fossils of invertebrates are found in various types of sediment from the Phanerozoic.
Fossils of invertebrates are commonly used in stratigraphy.
Jean-Baptiste Lamarck, who was appointed to the position of "Curator of Insecta and Vermes" at the Muséum National d'Histoire Naturelle in 1793, both coined the term "invertebrate" to describe such animals and divided the original two groups into ten, by splitting Arachnida and Crustacea from the Linnean Insecta, and Mollusca, Annelida, Cirripedia, Radiata, Coelenterata and Infusoria from the Linnean Vermes.
Significance of the group
Invertebrates are animals without a vertebral column.
This has led to the conclusion that invertebrates are a group that deviates from the normal, vertebrates.
This has been said to be because researchers in the past, such as Lamarck, viewed vertebrates as a "standard": in Lamarck's theory of evolution, he believed that characteristics acquired through the evolutionary process involved not only survival, but also progression toward a "higher form", to which humans and vertebrates were closer than invertebrates were.
Although goal-directed evolution has been abandoned, the distinction of invertebrates and vertebrates persists to this day, even though the grouping has been noted to be "hardly natural or even very sharp."
Another reason cited for this continued distinction is that Lamarck created a precedent through his classifications which is now difficult to escape from.
It is also possible that some humans believe that, they themselves being vertebrates, the group deserves more attention than invertebrates.
In any event, in the 1968 edition of Invertebrate Zoology, it is noted that "division of the Animal Kingdom into vertebrates and invertebrates is artificial and reflects human bias in favor of man's own relatives."
The book also points out that the group lumps a vast number of species together, so that no one characteristic describes all invertebrates.
In addition, some species included are only remotely related to one another, with some more related to vertebrates than other invertebrates (see Paraphyly).
For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" or charismatic species.
During the 20th century, invertebrate zoology became one of the major fields of natural sciences, with prominent discoveries in the fields of medicine, genetics, palaeontology, and ecology.
The study of invertebrates has also benefited law enforcement, as arthropods, and especially insects, were discovered to be a source of information for forensic investigators.
They have long been the most intensively studied model organisms, and were among the first life-forms to be genetically sequenced.
Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to animals.
Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change.
- Invertebrate zoology, classification of animals body symmetry
- Invertebrate paleontology
- Marine invertebrates
- Pain in invertebrates
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Invertebrate.