Invertebrate

The common fruit fly, Drosophila melanogaster, has been used extensively for research

Invertebrates are animal species that neither possess nor develop a vertebral column, derived from the notochord. By definition, this includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include insects, crabs, lobsters and their kin, snails, clams, octopuses and their kin, starfish, sea-urchins and their kin, and worms.

The overwhelming majority of animal species are invertebrates. One estimate in the journal Science put the figure at 97%.^[1] Many individual invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata^[2] and some of the so-called invertebrates, such as the Chaetognatha, Hemichordata, Tunicata and Cephalochordata are more closely related to the vertebrates than to other invertebrate phyla. This makes the term "invertebrate" almost meaningless for taxonomic purposes.

Etymology

The word "invertebrate" derives from the prefixed form of the Latin word vertebra. Vertebra means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. In turn the jointed aspect of vertebra derived from the concept of turning, expressed in the root verto or vorto, to turn.^[3] Coupled with the prefix in-, meaning "not" or "without",^[4] the word conveys the meaning: "those that lack vertebrae".

Taxonomic significance

The term Invertebrates generates a great deal of confusion among non-biologists; it does not refer to any particular taxon in the same way that for instance Arthropoda, Vertebrata or Manidae do. Each of those examples describes a (presumably monophyletically) valid taxon, say a phylum, subphylum or family. In referring to taxonomy of the Animalia, "invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except arguably within the Chordata. The Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of "Vertebrata" and "Invertebrata" would be about as practical as classifying animals into mayflies and non-mayflies, or transport into rowing boats and non-rowing boats. It would be logically correct to do so, and rowing boats as such do form a practical group, but speaking of "non-rowing boats" would lump together land, sea, air and space transport in ways that rarely would be useful. In formal taxonomy of the Animalia there are higher level 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^[5] or perhaps bauplan.^[6] The resultant cladistic structure would not resemble anything like a binary split into vertebrates and invertebrates.

At the same time, there certainly is no reason to avoid the use of the terms Invertebrata or invertebrates when they are convenient, but it is important not to confuse the terms with the names or relationships of actual taxa.

Of the million or more animal species in the world, more than 98% are invertebrates. Invertebrates don't have a skeleton of bone, either internal or external. They include hugely varied body plans. Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms. Others have hard exoskeletons, outer shells like those of insects and crustaceans. The most familiar invertebrates include the Protozoa, Annelida, Echinodermata, Mollusca and Arthropoda. Arthropoda include insects, crustaceans and arachnids.

Characteristics

The trait that is common to all invertebrates is the absence of a Porifera, invertebrates generally have bodies composed of differentiated tissues. There is also typically a digestive chamber with one or two openings to the exterior.

Like vertebrates, most invertebrates reproduce at least partly through sexual reproduction. They produce specialized reproductive cells that undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.^[7] These fuse to form zygotes, which develop into new individuals.^[8] Others are capable of asexual reproduction, or sometimes, both methods of reproduction.

Phyla

The fossil coral Cladocora from the Pliocene of Cyprus

The term invertebrates covers several phyla. One of these are the sponges (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.^[37] Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change.^[38]

Arthropods, especially insects, are often used by forensic scientists. For example, some invertebrates are attracted to dead bodies.^[18]

In research

Invertebrates are animals without a vertebral column. This has led to the conclusion that invertebrates are a group that deviates from the norm, vertebrates. This has been said to be due to the fact that 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's also possible that some humans believe that, they themselves being vertebrates, the group deserves more attention than invertebrates.^[35] 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.^[36]

Significance of the group

sea sponges and flatworms to complex animals such as arthropods and molluscs.

Classification

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.^[33] Fossils of invertebrates are commonly used in stratigraphy.^[34]

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.^[30] Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago.^[31] Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms.^[32]

History

• Protozoa (like the worms, an arbitrary grouping of convenience; link to their article for details)
• Sponges (Porifera)
• Stinging jellyfish
• Comb jellies
• Flatworms (Platyhelminthes)
• Round- or threadworms
• segmented worms
• Insects, spiders, crabs and their kin
• Cuttlefish, snails, mussels and their kin
• Starfish, sea-cucumbers and their kin

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.

Classification of invertebrates

Among lesser phyla of invertebrates are the Hemichordata, or acorn worms,^[29] and the Chaetognatha, or arrow worms. Other phyla include Acoelomorpha, Brachiopoda, Bryozoa, Entoprocta, Phoronida, and Xenoturbellida.

Also included are two of the most successful animal phyla, the Mollusca and Annelida.^[25][26] 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.^[27] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.^[28]

Another phylum is Platyhelminthes, the flatworms.^[21] These were originally considered primitive, but it now appears they developed from more complex ancestors.^[22] Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.^[23] The Rotifera or rotifers, are common in aqueous environments. Invertbrates also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and the Cycliophora.^[24]

. Sipuncula or ribbon worms, and the Nemertea. These groups have a reduced coelom, called a pseudocoelom. Other invertebrates include the Loricifera, and Priapulida, Kinorhyncha Smaller phyla related to them are the ^[20] A number are important parasites.^[19] 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.Nematoda, are close relatives of the arthropods and share these traits. The Tardigrada and Onychophora Two smaller phyla, the [18] The largest animal phylum is also included within invertebrates: the

The Echinodermata are radially symmetric and exclusively marine, including starfish (Asteroidea), sea urchins, (Echinoidea), brittle stars (Ophiuroidea), sea cucumbers (Holothuroidea) and feather stars (Crinoidea).^[17]

^[16].diploblastic There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, they are sometimes called [15]