Phylum Chordata

** Definition, Animals, and Characteristics


Definition


Phylum Chordata is the largest phylum of the superphylum Deuterostome (the other phylum within this group is Echinodermata).

Members of this group, Chordata, are generally referred to as chordates. They include a wide variety of organisms that share four fundamental characteristics namely, a notochord, a dorsal tubular nerve cord, pharyngeal gill-slits, and a post-anal tail (in human beings, this only exists during the embryonic stage)

Currently, the phylum Chordata is estimated to consist of over 60,000 species. They vary in size, shape, and weight, and can be found in different environments and habitats across the globe (marine and freshwater, air, land, poles).




Although they share three main characteristics, some of the other minor characteristics used to identify chordates include:


  •        Bilateral symmetry
  •        A true coelom
  •        They are triploblastic
  •        Axiate organization
  •        Metamerism

 

* These minor characteristics can also be found in some non-chordates, especially higher non-chordates.

 

Chordates include:


  •        Mammals
  •        Reptiles
  •        Amphibians
  •        Fish
  •        Birds
  •        Lancelets


Phylum Chordata: The Four Main Characteristics


Before examining the different types of animals in this group and their respective traits, it's important to understand the four fundamental features and their general functions:


Notochord


The notochord is located slightly above the alimentary canal/gastrointestinal tract and beneath the dorsal tubular nerve cord. It's a rod-like structure that consists of elastic cartilage.

Cells of the notochord (notochordal cells) are large in size and vacuolated. They are also enclosed within a fibrous sheath of connective tissue.

In some animals, the notochord acts as a primitive internal skeleton that not only provides support but also allows for body movement. In animals like lancelets, the notochord persists through all the stages of development. In others, however, it might be partially or completely replaced by the spinal column/backbone. In human beings, for instance, the notochord is reduced to cartilage disks located between the vertebrae and thus not completely lost.



Dorsal Tubular/Hollow Nerve Cord


The dorsal tubular nerve cord consists of nerve fibers and is located slightly above the anteroposterior axis. It also consists of a cavity known as neurocoel that is filled with fluid.

In chordates, this feature gives rise to the central nervous system. Whereas the anterior of this feature develops to form the brain (cerebral vesicle), the posterior segment develops into the spinal cord.

 

* Nonchordates generally have a ventral/double nerve cord in place with a cluster of nerve cells known as ganglia.


Pharyngeal Gill-Slits


Pharyngeal gill-slits are also known as gill-clefts or branchial clefts. They are located on either side just behind the mouth, they are the openings that lead from the pharynx of the organism to the exterior part (outside). In various aquatic organisms, they are lined with vascular lamellae forming structures known as gills used for respiration.

In fish, they line the gills and allow water in and out (for respiration). However, they are also used for filter-feeding by some deuterostomes (some lower chordates).

Like many other chordates, these gill slits are also present in human beings during embryonic development. The tissue, however, develops in areas around the ears, jaws, and other parts of the neck.


Post-Anal Tail


This is posterior elongation that extends past the anus in the majority of chordates. Consisting of skeletal material and muscle, the tail serves different functions in different animals.

For instance, while it helps animals like dogs and cats maintain balance, it allows fish and some aquatic organisms to swim. In some chordates like human beings, the post-anal tail is only present in the embryonic stage before it is lost.


Phylum Chordata: Animals and their Characteristics


Members of the phylum Chordata are widely divided into two main categories namely, protochordates and higher chordates.


Protochordata


Protochordata, which consists of lower chordates, is an informal group that largely consists of invertebrate chordates.

As members of the phylum Chordata, these organisms possess a notochord at some stage of their development. However, they do not develop a backbone or vertebral column found in various higher animals. In some of the species, the notochord persists throughout.

The majority of Protochordata are marine organisms that can be found living in colonies or individually. While they share all characteristics associated with the phylum Chordata, invertebrate chordates largely rely on plankton and various suspended organic matter as a source of nutrition. 

 

Protochordata is divided into two groups or subphyla that include:



Tunicata (Urochordata)


The subphylum Tunicata consists of tiny marine animals known as tunicates or sea squirts. Like other members of the group Protochordata, tunicates are exclusively marine.

In this environment, the majority of species can be found attached to various surfaces including rocks, seaweed, other marine animals, as well as docks and the lower surfaces of boats among others. Some, like members of the class Thaliacea, however, can swim freely while others (e.g. Larcavea species) are free-floating.

 

* The name "Tunicate" is derived from the word "Tunic" which describes the flexible covering on their surface.


* Currently, there are an estimated 3,000 species in the subphylum with the majority being members of the class Ascidiacea.

 

Most species in this group are characterized by a blob or sac-like body structure (adult forms) within a test/tunic. They are also mostly transparent or translucent with a varying number of gill slits.

With the exception of some Larvacea species, most Tunicata species only possess the nerve cord and notochord during their larval stage. In this species, the notochord is located in the tail segment of the body which is lost as they develop into adults. Some members of Larvacea, on the other hand, retain most of their larval features in adulthood and thus retain the notochord a little longer than the other species.



Some species of the subphylum Tunicata include:


  •        Pyrosoma
  •        Ciona
  •        Appendicularia
  •        Doliolum

 

The following is a diagrammatic representation of a tunicate (adult):


Tunicate Adult of phylum Chordata. Credit: MicroscopeMaster.comTunicate Adult of phylum Chordata. Credit: MicroscopeMaster.com


* As mentioned, most of these species feed on plankton and other organic material in their surroundings. To do this, they draw in water through the oral siphon so that food materials are trapped in the branchial basket. Water is then expelled through the atrial siphon.

 

Some of the other characteristics associated with tunicates include: 


  • They may have scattered muscles (e.g. in sessile tunicates) or circular muscle bands (e.g. Doliolum).
  • The number of gills-slits varies from two (e.g. in Appendicularia) to many (e.g. in Molgula).
  • They can reproduce sexually and asexually (E.g. most ascidian species have male and female gonads). In the class Larvacea, most of the species are hermaphrodites and therefore possess both the male and female gonads.



Cephalochordata


The subphylum Cephalochordata is a small group consisting of about 30 species. Unlike tunicates, members of this group retain the notochord and nerve cord throughout their life. For these species, the notochord extends the entire length of the body. This is different compared to urochordates where the notochord is only located in the tail segment of the organism.

Also known as lancelets (or amphioxus - meaning they have pointed ends), cephalochordates are characterized by a fish-like body (members of the class Leptocardii). However, they are small in size, averaging 2.5 inches in length. They are also characterized by fins with blocks of muscle known as metameres that allow them to swim from one point to another.

Aside from the notochord, cephalochordates also retain all the other main features associated with phylum Chordata when they mature into adults. Commonly found in shallow coastal waters (in the tropical and temperate regions), some of the species are free-swimmers while others are burrowers.

Like some of the other species, they are also filter feeders. Burrowers typically bury their bodies in the sand so that the head region remains suspended in water. When water washes over the pharynx region, it's sieved through cilia to retain organic matter and plankton.

 

* Adult lancelets can draw water into the mouth where cirri (located around the mouth) filters small organisms.





Some of the other characteristics of lancelets include:


  • As mentioned, lancelets are capable of quick/rapid movement on various surfaces (e.g. on gravel). This allows them to move to new favorite areas or escape predators.
  • Adult forms can also feed on larger organic material that settles on the sand.
  • Also referred to as acrania because they do not have a cranium.
  • Pharynx consists of many gill slits.
  • Notochord consists of a firm sheath.
  • The nerve cord consists of eyelike organs that allow the organism to detect changes in light intensity.
  • While they do not have a heart, the pattern of blood circulation slightly resembles that of vertebrates (they have a closed system).
  • They reproduce sexually - There are separate sexes and fertilization occurs externally.
  • Do not have an exoskeleton (tunic).

 

Some examples of Caphalochordata are

 

  • Asymmetron
  • Branchiostoma


Diagrammatic representation of a Lancelet in phylum Chordata. Credit: MicroscopeMaster.comDiagrammatic representation of a Lancelet in phylum Chordata. Credit: MicroscopeMaster.com


Hemichordata (Adelochordata)


Although this group was initially considered a subphylum of Protochordates within the phylum Chordata, recent findings do not support this position. For instance, based on several studies, hemichordates have been shown to possess a stomochord which is not a true notochord.

They are divided into three groups/classes which include Enteropneusts, Planctosphaeroidea, and Pterobranchs, and are generally characterized by a tubular, elongated body.

 

* The word Hemichordata is derived from the Greek words "Hemi" which means half and "chorde" which means string. The word therefore generally means half-chordate.

 

Their bodies are divided into three main regions which include the trunk, the collar, and the proboscis.


Some members of Hemichordata include:


  •   Balanoglossus
  •   Caphalodiscuss
  •   Rhabdopleura
  •   Ptychodera


Higher Chordates (Vertebrata)


The second category of the phylum Chordata belongs to the subphylum Vertebrata. In some literature, they are referred to as higher Chordates because they are more developed as compared to Protochordates. As the name suggests, these species are characterized by a vertebral column (backbone).

 

*  Vertebrata are also known as craniates (craniata) because they have a cranium (skull) consisting of bone and cartilage.

 

Like lower chordates, Vertebrata also possess a notochord at a certain point of their development (embryonic stage). The notochord, however, is partially or completely replaced by the spinal column as they develop.

As with the notochord, all the other major features associated with chordates are gradually replaced or lost as the organism develops. For instance, in human beings, the dorsal hollow nerve cord gives rise to the central nervous system while the post-anal tail feature is lost (the coccyx is the only remnant in this case). However, given that they possessed all these features at some point, they are regarded as chordates.

With over 45,000 species, Vertebrata is the largest subphylum in the phylum Chordata.


The body of these organisms can be divided into four main sections which include:


  •        Head
  •        Neck
  •        Trunk
  •        Tail (present in the majority of vertebrates)



Vertebrata Animals and their Characteristics


The subphylum Vertebrata is divided into two main divisions that include:

 

  • Agnatha
  • Gnathostomata


Agnatha


The division Agnatha consists of primitive jawless fish (and fish-like organisms). Some of the most popular members of this group are lampreys and hagfish.

Unlike some of the other chordates, members of this group have been described as some of the oldest vertebrates around. Aside from a true jaw, Agnatha species also lack paired limbs, a stomach, and immunoglobulins commonly found in jawed vertebrates. However, they have been shown to have a distinct type of adaptive immunity where variable lymphocyte receptors act as antibodies.

 

There are two main classes of the division Agnatha:



Ostracodermi


Ostracoderms can be traced back to the Paleozoic Era. Some of the species in this group are Caphalaspis and Thelodus. Also known as armored fish, ancient Ostracoderms lived in freshwater as bottom dwellers.

Though they had fish-like bodies, most were small in size, not exceeding 30 centimeters in length. As the name suggests, they possessed heavy armor that covered the head and thorax region of the body for protection (against some arthropods and eurypterids).

 

Some of the other features that were found in these organisms included:

 

  •        Significant number of gills.
  •        Mouth located on the ventral side of the body.
  •        Pair of lateral eyes as well as a median pineal eye.

 


Cyclostomata (Cyclostomi)


Cyclostomata species are characterized by an eel-like body. Like Ostracodemi, they also lack a true jaw and lateral fins. Some members of this group are Myxine and Petromyzon.

 

Some of the other characteristics of these species include:


  •        Rounded mouth
  •        5 to 16 pairs of gills
  •        A heart with two chambers


Gnathostomata


Members of this division have a true jaw and paired limbs. The division is divided into two superclasses namely, Pisces and Tetrapoda.



Pisces


The superclass Pisces consists of fish and fish-like organisms that have a true jaw and paired fins (for locomotion). They also characterized median fins, gills on either side of the body behind the mouth (5 to 7 pairs of gill slits), as well as scaly skin.

Like many other fishes, Pisces are aquatic and can be found in both marine and freshwater environments. They have a heart with two chambers (an auricle and ventricle), a brain, and an internal ear with three semicircular canals. The sexes are separate and fertilization may occur internally or externally.

 

Pisces are further divided into several classes that include:

 

Placodermi - Placodermi is an extinct group that can be existed during the Permian period of the Paleozoic Era. Generally, they were characterized a bony exoskeleton armor covering the head, an endoskeleton, hyoidean gill-slits, a caudal fin, and a primitive jaw. Some examples of these species are Dinichthys and Climatius.

 

Chondrichthyes - Also known as Elasmobranchi, Chondrichthyes are marine fishes that have placoid scales for an exoskeleton. Also in present are gill-slits (without the operculum), a mouth, male and female sexes (males have claspers), as well as ventral nostrils. Some examples of Chondrichthyes are Chimaera and Scoliodon.


Osteichthyes - Examples of Osteichthyes include Protopterus and Labeo. They can be found in both marine and freshwater environments and may have cycloid, placoid, or ctenoid scales. The gill slits in these species are covered with an operculum and the sexes are divided into male and female (males also have claspers).


Tetrapoda


The term tetrapoda generally refers to chordates with four limbs. Unlike Pisces, which are commonly found in aquatic habitats, Tetrapoda can be found in different environments across the globe.

 

Tetrapoda is divided into four main classes that include:


Amphibia - Amphibians can be found in both aquatic and terrestrial habitats. Some of the most common examples include frogs, salamanders, and toads among others.

Generally, amphibians can be found in aquatic habitats as larva where they breathe by use of gills. Adults, on the other hand, can be found on land where and use their lungs for respiration. However, adult frogs also perform respiration through the skin and thus need to keep moist. Their heart consists of three chambers and serves to pump oxygen-rich blood throughout the body.

 

Reptilia - Like frogs, reptiles can also be found in aquatic and terrestrial environments. The skin of most reptiles is dry and covered by horny scales. Unlike frogs, their hearts consist of 4 chambers.

 

*  Though snakes don't have four limbs, they are descendants of animals with four limbs.


*  Though they can also be found in aquatic environments, reptiles do not have a larval aquatic phase.

 

Aves (Birds) - Unlike the other organisms in this group, birds are covered with feathers. Whereas hind limbs are used for walking or swimming, the forelimbs are modified into wings that allow them to fly. Like mammals, birds are warm-blooded and thus have a given constant body temperature. They also have a heart with four chambers as well as lungs with air-sacs where oxygen and carbon dioxide are exchanged during respiration.

 

Mammalia - Mammalians have skin consisting of various glands. The majority of mammals are also covered with hair. One of the main characteristics of these organisms is that the female possesses mammary glands to suckle the young. Like birds, they also have a heart with four chambers as well as a lung with air-sacs. Elephants, rats, and human beings are some examples of mammals.

 

*  Most mammals give birth to young ones. However, there are a few that lay eggs. These are the duck-billed platypus and the spiny anteater.


*  Though the majority of mammals can be found on land, others like whales and seals can be found in aquatic environments. 

See also: Zebrafish


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References


Ahmad, T. General Character and Classification of Pisces. Department of Zoology, BSNV PG College, Lucknow.

Matsushita, M. (2018). The Complement System of Agnathans. Department of Applied Biochemistry, Tokai University, Hiratsuka, Japan.

Verma, P. S. (2012). Chordate Zoology.

 

 

Links


https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/agnatha

https://www.researchgate.net/publication/350823166_Vertebrates_Chordata






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