Phylum Chordata include the animals from the vertebrates, the tunicates, lancelets, and mammals, the birds, reptiles, fishes and amphibians. The latter ones are the most common and familiar ones.
Definition:
This is a phylum named as Chordata. The animal in it are known as chordate. These animals have similar characteristics and this is evident during the larva or adulthood period. This particular trait makes them different from others or different from other phyla or taxa. These special characters includes a notochord, nerve cor, thyroid, pharyngeal slits and post anal tail. The chordates got their names from notochord characteristic which is very important in the formation of their structure and also is pivotal in the formation of their body. They are symmetrical in their shape and have a circulatory system and also show acute segmentation.
Chordates are known to be defined by their morphological characters but they can also be distinguished by genome sequence.The fossils of all the chordates have been found as early as five hundred million years ago. The vertebrates Chordata with the notochord is changed by a vertebral column when they are developing and they are also considered to be in the group of Craniata which are nothing but chordates with a skull.
The Classification for the Chordata goes as follows:
It is further brown down into the :
- Subphylum Cephalochordata( which are Acraniata) these are about thirty species.
- Subphylum Tunicata- which are also Urochordata. These are about three thousand species. These are further classified into the classes- Class Ascidiacea which are Sea species. Class Thaliacea which are the salps.Third is the Class Appendicularia and lastly the Class Sorberacea.
- Subphylum Craniata which are vertebrates and has about sixty six thousand species. They have a superclass under them known as Agnatha with over hundred species.
The hemichordate has been given as the fourth chordate subphylum but now they are treated as a complete separate class. These are the acorn worms and they are placed along with Echinodermata which form the Ambulacraria which is the sister phylum of Chordates. Of the species more than sixty five thousand living species are that of Chordates and they are bony fish types which are member of the superclass pisces.
Some of the Craniates have distinct skulls and this group will include the hagfish which does not have a vertebrae. They are characterized by the heads, same like the chordates or the others by their tail structure. Most of the craniates are vertebrates where the notochord is replaced by the vertabral column. These have bony or cartilaginous vertebrae with some neural arches that does protect the spinal cord. The hagfishes have incomplete brain formation and absolutely no vertebrae and thus they are not included also in the vertebrates. They are a member of the Craniates and thus over time has evolved. However the hagfish is excluded from the vertebrates or not is a matter of debate and much talk and there have also been talk that these might be degenerate vertebrates who do not have vertebral columns.
Phylogeny: There have been researches going on to verify and found out the earliest form of the common Chordates. The history goes back to the earliest forms of Chordates which do not have a backbone and notochord. These structures might have been lost over time and thus complicating the chordates classification. There have been some chordates lineages that have been found and by DNA analysis there have been assumptions that there was no chordate like structures before.
The evolution of the chordates has been traced back to the earlier times that have contributed to several hypothesis. The current chordates are uni phyletic and this means the chordates come from a single ancestor which is also a chordate. The earliest forms of Chordates were the two species that are actually the fish and which also says that the they are vertebrates.There is also a difficulty in classifying the chordates which are living and there are attempts to make the family tree and others to show they belong to a class which is para phyletic.
In this Phylum the majority animals are complex than the jellyfish and others and they are divided into two groups- the protostomes and deters tomes. The latter one had majority of the chordates. In the ancient times the Kimberella was a member of the protostomes and thus they were of the deters tome lineage and it was discovered ancient times, many years ago.
Do you know the answer of this question: Which of the following is not a protochordate?
- A) Herdmania
- B) Amphioxus
- C) Salpa
- D) Petromyzon
