3 subphyla of chordates
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. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger cladeslike Ecdysozoa and Embryophyta.
Introduction
Diese Summe nennen wir Stamm Phylon. We name this aggregate [a] Stamm [i. In plant taxonomyAugust W. Eichler classified plants into five groups named divisions, a term that remains in use today for groups of article source, algae and fungi. Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan. 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.
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 the Pogonophora in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelidsso the phyla were merged the bearded worms are now an annelid family. This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of cladisticsa method in which groups are placed on a "family tree" without any formal 3 subphyla of chordates of group size. 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, 3 subphyla of chordates phylum is defined by a set 3 subphyla of chordates 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.
Habitat and life cycle
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. 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 3 subphyla of chordates diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary here, not least because some are based on described species, [14] some on extrapolations to numbers of undescribed species.
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For instance, around 25,—27, species of nematodes have been described, while published estimates of the total number of nematode species include 10,—20,; ,; 10 million; and million. The kingdom Plantae is defined in various ways by different biologists see Current definitions of Plantae.
All definitions include the living embryophytes land plantsto which may be added the two green algae divisions, Chlorophyta and Charophytato form the clade Viridiplantae. The table below follows the influential though contentious Cavalier-Smith system in equating "Plantae" with Archaeplastida subhpyla, [19] a group containing Viridiplantae and the algal Rhodophyta and 3 subphyla of chordates divisions.
The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years.
Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, [20] while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms i. Since the first publication of the APG system inwhich proposed a classification of angiosperms up to the level of ordersmany sources have preferred to treat ranks higher than orders as 3 subphyla of chordates clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e. Chorrates [27]. Hepatophyta [23].]
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