Invertebrates

Invertebrates are animal species that do not possess or develop a vertebral column, derived from the notochord. This in effect includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include insects, worms, clams, crabs, octopus, snails, and starfish. Taxonomically speaking, “invertebrate” is no more than a term of convenience. The overwhelming majority of animal species are invertebrates, because only about 4% of animal species include a vertebral column in their anatomy. In other words all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals) are regarded as invertebrates. Furthermore, many individual invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata. In fact some of the so-called invertebrata, such as the Chaetognatha and Hemichordata, are more closely related to the Chordata than to other invertebrate phyla.

Invertebrates form a massively paraphyletic group. It is generally accepted that the phyla comprising modern Metazoa share a common multicellular ancestor, but with the sole exception of one subphylum of the phylum Chordata, all those phyla are classified as invertebrates along with two of the three subphyla in the Chordata: Tunicata and Cephalochordata. These two, plus all the other known invertebrates, have only one cluster of Hox genes, while the vertebrates have duplicated their original cluster more than once.Within palaeozoology and palaeobiology, invertebrates are often studied within the fossil discipline called invertebrate paleontology.

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. 

The trait that is common to all invertebrates is the absence of a vertebral column: 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.

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.

The term invertebrates covers several phyla. One of these are the sponges (Porifera). They were long thought to have diverged from other animals early.[9] They lack the complex organization found in most other phyla.[10] Their cells are differentiated, but in most cases not organized into distinct tissues.[11] Sponges typically feed by drawing in water through pores.[12] Some speculate that sponges are not so primitive, but may instead be secondarily simplified.[13] 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.[14] Both have distinct tissues, but they are not organized into organs.[15] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, they are sometimes called diploblastic.[16]

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]

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

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]

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

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