Phylum

In biology, a phylum is a level of classification, or taxonomic rank, that is below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 32 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about eight phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta.

General description

The term phylum was coined in 1866 by Ernst Haeckel from the Greek , related to . Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as a group : "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate Stamm ." In plant taxonomy, August W. Eichler classified plants into five groups named divisions, a term that remains in use today for groups of plants, algae and fungi.
The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.
At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity, or a group of organisms with a certain degree of evolutionary relatedness. Attempting to define a level of the Linnean hierarchy without referring to relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were.

Definition based on genetic relation

The most important objective measure in the above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine the relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids, so the phyla were merged. On the other hand, the highly parasitic phylum Mesozoa was divided into two phyla when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes.
This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size.

Definition based on body plan

A definition of a phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen. The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by a set of characters shared by all its living representatives.
This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on a subjective decision about which groups of organisms should be considered as phyla.
The approach is useful because it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.
A classification using this definition may be strongly affected by the chance survival of rare groups, which can make a phylum much more diverse than it would be otherwise.

Known phyla

Animals

Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, and some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.

Phylum	Meaning	Common name	Distinguishing characteristic	Taxa described
Agmata	Fragmented		Calcareous conical shells	5 species, extinct
Annelida	Little ring	Segmented worms, annelids	Multiple circular segments	22000+ extant
Arthropoda	Jointed foot	Arthropods	Segmented bodies and jointed limbs, with chitin exoskeleton	1250000+ extant; 20,000+ extinct
Brachiopoda	Arm foot	Lampshells	Lophophore and pedicle	300–500 extant; 12,000+ extinct
Bryozoa	Moss animals	Moss animals, sea mats, ectoprocts	Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia	6000 extant
Chaetognatha	Longhair jaw	Arrow worms	Chitinous spines either side of head, fins	extant
Chordata	With a cord	Chordates	Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail	+
Cnidaria	Stinging nettle	Cnidarians	Nematocysts
Ctenophora	Comb bearer	Comb jellies	Eight "comb rows" of fused cilia	–150 extant
Cycliophora	Wheel carrying		Circular mouth surrounded by small cilia, sac-like bodies	3+
Dicyemida	Lozenge animal		Single anteroposterior axial celled endoparasites, surrounded by ciliated cells	100+
Echinodermata	Spiny skin	Echinoderms	Fivefold radial symmetry in living forms, mesodermal calcified spines	extant; approx. 13,000 extinct
Entoprocta	Inside anus	Goblet worms	Anus inside ring of cilia
Gastrotricha	Hairy stomach	Hairybellies	Two terminal adhesive tubes
Gnathostomulida	Jaw orifice	Jaw worms	Tiny worms related to rotifers with no body cavity
Hemichordata	Half cord	Acorn worms, hemichordates	Stomochord in collar, pharyngeal slits	extant
Kinorhyncha	Motion snout	Mud dragons	Eleven segments, each with a dorsal plate
Loricifera	Armour bearer	Brush heads	Umbrella-like scales at each end
Micrognathozoa	Tiny jaw animals		Accordion-like extensible thorax	2
Mollusca	Soft	Mollusks/molluscs	Muscular foot and mantle round shell	85000+ extant; 80,000+ extinct
Monoblastozoa	One sprout animals		distinct anterior/posterior parts and being densely ciliated, especially around the "mouth" and "anus".	1
Nematoda	Thread like	Roundworms, threadworms, eelworms, nematodes	Round cross section, keratin cuticle	25000
Nematomorpha	Thread form	Horsehair worms, Gordian worms	Long, thin parasitic worms closely related to nematodes
Nemertea	A sea nymph	Ribbon worms	Unsegmented worms, with a proboscis housed in a cavity derived from the coelom called the rhynchocoel
Onychophora	Claw bearer	Velvet worms	Worm-like animal with legs tipped by chitinous claws	extant
Orthonectida	Straight swimmer		Parasitic, microscopic, simple, wormlike organisms	20
Petalonamae	Shaped like leaves		An extinct phylum from the Ediacaran. They are bottom-dwelling and immobile, shaped like leaves, feathers or spindles.	3 classes, extinct
Phoronida	Zeus's mistress	Horseshoe worms	U-shaped gut	11
Placozoa	Plate animals	Trichoplaxes, placozoans	Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between	4+
Platyhelminthes	Flat worm	Flatworms	Flattened worms with no body cavity. Many are parasitic.
Porifera	Pore bearer	Sponges	Perforated interior wall, simplest of all known animals	10800 extant
Priapulida	Little Priapus	Penis worms	Penis-shaped worms
Proarticulata	Before articulates		An extinct group of mattress-like organisms that display "glide symmetry." Found during the Ediacaran.	3 classes, extinct
Rotifera	Wheel bearer	Rotifers	Anterior crown of cilia
Saccorhytida	Saccus : "pocket" and "wrinkle"		Saccorhytus is only about 1 mm in size and is characterized by a spherical or hemispherical body with a prominent mouth. Its body is covered by a thick but flexible cuticle. It has a nodule above its mouth. Around its body are 8 openings in a truncated cone with radial folds. Considered to be a deuterostome or an early ecdysozoan.	2 species, extinct
Tardigrada	Slow step	Water bears, moss piglets	Microscopic relatives of the arthropods, with a four segmented body and head	1000
Trilobozoa	Three-lobed animal	Trilobozoans	A taxon of mostly discoidal organisms exhibiting tricentric symmetry. All are Ediacaran-aged	18 genera, extinct
Vetulicolia	Ancient dweller	Vetulicolians	Might possibly be a subphylum of the chordates. Their body consists of two parts: a large front part and covered with a large "mouth" and a hundred round objects on each side that have been interpreted as gills or openings near the pharynx. Their posterior pharynx consists of 7 segments.	15 species, extinct
Xenacoelomorpha	Strange hollow form	Xenacoelomorphs	Small, simple animals. Bilaterian, but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems	400+
Total: 39				1,525,000