Invertebrate

An invertebrate is an animal (a multi-cellular eukaryote) without a backbone. The group includes 97% of all animal species[1] – all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals).

Invertebrates form a paraphyletic group. Given a common multicellular, eukaryotic ancestor, all contained phyla are invertebrates along with two of the three subphyla in Phylum Chordata: Urochordata 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 palaeontology.

The trait that is common between all invertebrates is the absence of a backbone: this creates a distinction between invertebrates, and vertebrates. Being animals, invertebrates are heterotrophs, and require sustenance in the form of consuming other organisms. Their cells also lack rigid cell walls. 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 in it.

Many invertebrates reproduce through sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.[3] These fuse to form zygotes, which develop into new individuals.[4] Others are capable of asexual reproduction, or sometimes, both methods of reproduction.
[edit] Phyla
The fossil coral Cladocora from the Pliocene of Cyprus

The term invertebrate covers several phyla. One of these are the sponges (Porifera). They were long thought to have diverged from other animals early.[5] They lack the complex organization found in most other phyla.[6] Their cells are differentiated, but in most cases not organized into distinct tissues.[7] Sponges typically feed by drawing in water through pores.[8] Some speculate that sponges are not so primitive, but may instead be secondarily simplified.[9] 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.[10] Both have distinct tissues, but they are not organized into organs.[11] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, they are sometimes called diploblastic.[12]

The Echinodermata are radially symmetric and exclusively marine, including starfish, sea urchins, and sea cucumbers.[13] Other phyla of invertebrates are the Hemichordata, or acorn worms,[14] and the Chaetognatha, or arrow worms.

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.[15] 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.[16] A number are important parasites.[17] 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.[18] These were originally considered primitive, but it now appears they developed from more complex ancestors.[19] Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.[20] The Rotifera or rotifers, are common in aqueous environments. Invertbrates also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and the Cycliophora.[21]

Also included are two of the most successful animal phyla, the Mollusca and Annelida.[22][23] 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.[24] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.[25]

Other phyla include Acoelomorpha, Brachiopoda, Bryozoa, Entoprocta, Phoronida, and Xenoturbellida.

From : http://en.wikipedia.org/wiki/Invertebrate