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Invertebrates are a
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. Invertebrates vary widely in size, from 50 μm (0.002 in) rotifers to the 9–10 m (30–33 ft) colossal squid.
Some so-called invertebrates, such as the Tunicata and Cephalochordata, are more closely related to vertebrates than to other invertebrates. This makes the invertebrates paraphyletic, so the term has little meaning in taxonomy.
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".
The term invertebrates is not always precise among non-biologists since it does not accurately describe a
Despite this, the concept of invertebrates as a taxon of animals has persisted for over a century among the
Number of extant species
By far the largest number of described invertebrate species are insects. The following table lists the number of described
|Invertebrate group||Phylum||Image||Estimated number of|
sponges, other worms etc.
The trait that is common to all invertebrates is the absence of a
Morphology and symmetry
Other examples of asymmetry are found in
One type of invertebrate respiratory system is the open
A tracheal tube may contain ridge-like circumferential rings of
Like vertebrates, most invertebrates reproduce at least partly through
Social behavior is widespread in invertebrates, including cockroaches, termites, aphids,
The term invertebrates covers several phyla. One of these are the sponges (
The largest animal phylum is also included within invertebrates: the Arthropoda, including insects,
Another phylum is
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
Classification of invertebrates
Invertebrates can be classified into several main categories, some of which are
- Comb jellies(Ctenophora)
- Medusozoans and corals (Cnidaria)
- Acoels (Xenacoelomorpha)
- Flatworms (Platyhelminthes)
- Bristleworms, earthworms and leeches (Annelida)
- Insects, springtails, crustaceans, myriapods, chelicerates (Arthropoda)
- Chitons, snails, slugs, bivalves, tusk shells, cephalopods (Mollusca)
- Roundworms or threadworms(Nematoda)
- Scalidophores (Scalidophora)
- Lophophorates (Lophophorata)
- Velvet worms(Onychophora)
- Arrow worms(Chaetognatha)
- Gordian worms or horsehair worms(Nematomorpha)
- Ribbon worms (Nemertea)
- Starfishes, sea urchins, sea cucumbers, sea lilies and brittle stars (Echinodermata)
- Acorn worms, cephalodiscids and graptolites(Hemichordata)
- Salps, pyrosomes, doliolids, larvaceans and sea squirts(Tunicata)
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.
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.
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
Two of the most commonly studied model organisms nowadays are invertebrates: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. They have long been the most intensively studied model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but many genes, introns, and linkages have been lost. 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.
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